I took fudgy brownies, topped them with a thin chocolate frosting and sprinkles, and created a Birthday Dessert For A Crowd whose surprising cake-like interior will make readers curious about how I did it.

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I know a lot of birthday sweets, but these Brownie Birthday Cake Bars caught me off guard. Fudgy, soft center topped with a thin chocolate frosting and a scatter of rainbow sprinkles makes every bite feel a little rebellious.

The melted unsalted butter gives them that rich, almost boxed-cake-meets-homemade texture I can never resist. I brought them to a party once and they disappeared before anyone finished their drink.

They’re perfect if you want Celebration Brownies that steal the show, and they sit pretty among Homemade Birthday Desserts that people actually talk about later. I won’t spill every secret, but try one.

Ingredients

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Unsalted butter: Gives rich, silky texture, adds fat for fudgy brownies, high in calories.
Granulated sugar: Sweetens and helps crisp edges, carbs provide quick energy, no fiber.
Light brown sugar: Adds molasses depth, moistness and chew, slightly more complex sweetness.
Eggs: Bind, build structure, add protein and moisture, help rise a little.
Unsweetened cocoa powder: Gives chocolate flavor, low fat, adds bitterness balancing overall sweetness.
All-purpose flour: Provides structure, carbs and some protein, too much makes bars cakey.
Vanilla extract: Tiny splash lifts flavors, adds aroma and perceived sweetness without sugar.
Rainbow sprinkles: Purely decorative, adds crunch and color, mostly sugar so not nutritious.
Powdered sugar for frosting: Makes glossy frosting, provides bulk and sweetness, can be grainy if humid.

Ingredient Quantities

1 cup (2 sticks) unsalted butter, melted

2 cups granulated sugar

1/2 cup packed light brown sugar

4 large eggs

1 tablespoon vanilla extract

1 cup all-purpose flour

3/4 cup unsweetened cocoa powder

1/2 teaspoon fine salt

1/3 cup unsalted butter, melted (for frosting)

1/4 cup unsweetened cocoa powder (for frosting)

1 1/2 cups powdered sugar (for frosting)

2 tablespoons milk or cream (for frosting)

1/2 teaspoon vanilla extract (for frosting)

1/4 to 1/3 cup rainbow sprinkles

How to Make this

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1. Preheat oven to 350°F and line a 9×13 inch pan with parchment leaving an overhang, then lightly butter or spray the paper so it don’t move.

2. In a large bowl whisk together 1 cup melted unsalted butter, 2 cups granulated sugar and 1/2 cup packed light brown sugar until smooth.

3. Add 4 large eggs one at a time, mixing after each, then stir in 1 tablespoon vanilla extract.

4. Sift or whisk together 1 cup all-purpose flour, 3/4 cup unsweetened cocoa powder and 1/2 teaspoon fine salt; fold into the wet mix just until combined, don’t overmix or you’ll get cakey brownies.

5. Spread the batter evenly into the prepared pan, smooth the top with a spatula.

6. Bake 25 to 30 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs but not raw batter; let cool in the pan on a wire rack until room temperature, about 1 hour.

7. Make the thin chocolate frosting: melt 1/3 cup unsalted butter in a small saucepan or microwave, whisk in 1/4 cup unsweetened cocoa powder until smooth, then stir in 1 1/2 cups powdered sugar, 2 tablespoons milk or cream and 1/2 teaspoon vanilla extract; if too thick add milk 1 teaspoon at a time until it’s pourable but not runny.

8. Pour or spread a thin layer of the frosting over the cooled brownies, using an offset spatula for an even coat; if you want it extra thin warm the frosting slightly so it spreads easier.

9. Immediately sprinkle 1/4 to 1/3 cup rainbow sprinkles over the frosting, press lightly so they stick, then chill 15 to 30 minutes to set.

10. Lift the bars out using the parchment overhang, cut into squares with a sharp knife (wipe blade between cuts for clean edges), store airtight at room temp for 2 days or refrigerate for longer.

Equipment Needed

1. 9×13 inch baking pan lined with parchment paper (leave an overhang)
2. Large mixing bowl
3. Small saucepan or microwave-safe bowl for melting butter and making frosting
4. Measuring cups and measuring spoons
5. Whisk
6. Rubber spatula for folding batter
7. Offset spatula for spreading the frosting evenly
8. Fine-mesh sieve or sifter for cocoa and flour
9. Wire cooling rack and a sharp knife for clean cuts (wipe blade between slices)

Note: don’t forget a little butter or nonstick spray to keep the parchment from shifting.

FAQ

Q: What pan size should I use and how long do they bake? A: Use a 9×13 inch pan lined with parchment for easy removal. Bake at 350°F (175°C) about 25 to 30 minutes for fudgy bars, 30 to 35 minutes for more cake like. Edges should look set and a toothpick in the center will come out with a few moist crumbs not bone dry.

Q: Can I use salted butter or swap in oil? A: Yes salted butter works, just cut the added salt or skip it. You can swap melted butter 1:1 with a neutral oil for the batter if you want extra fudginess but the frosting needs butter so dont replace that.

Q: How do I get fudgier vs cakier bars? A: For fudgy, avoid overmixing, use the listed flour amount, and pull them out when the center still has moist crumbs. For cakier, add 2 tablespoons more flour and bake a few minutes longer until the toothpick is mostly clean.

Q: Any tips to make the frosting silky not grainy? A: Sift the powdered sugar and cocoa first, melt the butter then let it cool slightly before whisking in sugar, add milk 1 teaspoon at a time to reach spreadable texture. If it still seems grainy, beat it longer or add a teaspoon hot water and beat until smooth.

Q: How do I add sprinkles without them bleeding or sliding off? A: Spread the frosting while it is still a bit tacky then press sprinkles gently into the surface so they set. If colors bleed, chill the frosted bars for 10 minutes then add sprinkles. Put parchment under the pan to catch excess sprinkles for easy cleanup.

Q: Can I make these ahead or freeze them? A: Yes. Store unfrosted bars at room temp up to 2 days, or frosted in fridge up to 5 days. Freeze unfrosted bars wrapped well up to 3 months, thaw then frost. You can also freeze individual frosted squares layered with parchment for up to 2 months, just thaw in the fridge overnight.

Brownie Birthday Cake Bars Recipe Substitutions and Variations

Unsalted butter (1 cup or 1/3 cup for frosting) – Swap with melted coconut oil, 1:1 by volume. Coconut gives a faint coconut taste, and for the frosting chill it so it firms up. You can also use vegan stick margarine 1:1 for same texture.
Granulated sugar (2 cups) – Use coconut sugar 1:1 for a less refined alternative. If using honey or maple syrup, use about 3/4 cup liquid sweetener per 1 cup sugar and reduce other liquids slightly, brownies will be a bit more moist.
All-purpose flour (1 cup) – Substitute a 1:1 gluten-free flour blend straight across for gluten free brownies. If you want to try almond flour, use about 3/4 cup almond flour plus an extra binder (one small egg or a tbsp ground flax + water) since texture will be denser.
Eggs (4 large) – For a vegan option replace each egg with 1 tbsp ground flaxseed + 3 tbsp water (mix and let sit 5 min), or use 1/4 cup applesauce per egg for a moist, slightly cakier result.

Pro Tips

1. Let the butter and eggs sit at room temp for about 30 minutes before you mix them, they blend way easier and the batter comes together smoother so the brownies stay fudgy not cakey.

2. Fold the dry stuff in gently and stop when you still see a few streaks of flour, overmixing is the usual culprit for dry brownies, trust me just stop early.

3. If you want a super thin shiny frosting warm it just a little and add milk one teaspoon at a time until it barely pours, warming helps it level out and wont make it gloopy.

4. Sprinkle right after you spread the frosting and press them down lightly so they stick, then chill briefly to set; for neat squares wipe the knife between cuts with a cloth warmed under hot water.

Brownie Birthday Cake Bars Recipe

Recipe by Samantha Dalling

0.0 from 0 votes

Servings

servings

Calories

521

kcal

Equipment: 1. 9×13 inch baking pan lined with parchment paper (leave an overhang)
2. Large mixing bowl
3. Small saucepan or microwave-safe bowl for melting butter and making frosting
4. Measuring cups and measuring spoons
5. Whisk
6. Rubber spatula for folding batter
7. Offset spatula for spreading the frosting evenly
8. Fine-mesh sieve or sifter for cocoa and flour
9. Wire cooling rack and a sharp knife for clean cuts (wipe blade between slices)

Note: don’t forget a little butter or nonstick spray to keep the parchment from shifting.

Ingredients

1 cup (2 sticks) unsalted butter, melted
2 cups granulated sugar
1/2 cup packed light brown sugar
4 large eggs
1 tablespoon vanilla extract
1 cup all-purpose flour
3/4 cup unsweetened cocoa powder
1/2 teaspoon fine salt
1/3 cup unsalted butter, melted (for frosting)
1/4 cup unsweetened cocoa powder (for frosting)
1 1/2 cups powdered sugar (for frosting)
2 tablespoons milk or cream (for frosting)
1/2 teaspoon vanilla extract (for frosting)
1/4 to 1/3 cup rainbow sprinkles

Directions

Preheat oven to 350°F and line a 9×13 inch pan with parchment leaving an overhang, then lightly butter or spray the paper so it don't move.
In a large bowl whisk together 1 cup melted unsalted butter, 2 cups granulated sugar and 1/2 cup packed light brown sugar until smooth.
Add 4 large eggs one at a time, mixing after each, then stir in 1 tablespoon vanilla extract.
Sift or whisk together 1 cup all-purpose flour, 3/4 cup unsweetened cocoa powder and 1/2 teaspoon fine salt; fold into the wet mix just until combined, don't overmix or you'll get cakey brownies.
Spread the batter evenly into the prepared pan, smooth the top with a spatula.
Bake 25 to 30 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs but not raw batter; let cool in the pan on a wire rack until room temperature, about 1 hour.
Make the thin chocolate frosting: melt 1/3 cup unsalted butter in a small saucepan or microwave, whisk in 1/4 cup unsweetened cocoa powder until smooth, then stir in 1 1/2 cups powdered sugar, 2 tablespoons milk or cream and 1/2 teaspoon vanilla extract; if too thick add milk 1 teaspoon at a time until it's pourable but not runny.
Pour or spread a thin layer of the frosting over the cooled brownies, using an offset spatula for an even coat; if you want it extra thin warm the frosting slightly so it spreads easier.
Immediately sprinkle 1/4 to 1/3 cup rainbow sprinkles over the frosting, press lightly so they stick, then chill 15 to 30 minutes to set.
Lift the bars out using the parchment overhang, cut into squares with a sharp knife (wipe blade between cuts for clean edges), store airtight at room temp for 2 days or refrigerate for longer.

Notes

Below you’ll find my best estimate of this recipe’s nutrition facts. Treat the numbers as a guide rather than a rule—great food should nourish both body and spirit. Figures are approximate, and the website owner assumes no liability for any inaccuracies in this recipe.