Birthday Cake Blondies with Cream Cheese Frosting

These Birthday Cake Blondies are like a birthday party in every bite, chewy, rich, and loaded with melty white chocolate chips and rainbow sprinkles. Topped with a silky vanilla bean cream cheese frosting, they’re the ultimate treat for celebrations, bake sales, or just a little weekday joy. Whether you’re new to baking or a seasoned home baker, this easy recipe walks you through every step and explains the why behind the ingredients, so your blondies turn out perfect every time.

Ingredient Breakdown (and Why They Matter)

For the Blondies:

½ cup unsalted butter (melted)
Melted butter creates a soft, chewy blondie with that rich, almost caramelized flavor. Unsalted allows you to control the salt content.
1 cup brown sugar
Brown sugar adds moisture and chew, thanks to its molasses content. It also brings a warm, deep sweetness that complements the white chocolate beautifully.
2 eggs (room temperature)
Eggs bind everything together and provide structure and richness. Room temp eggs mix more evenly into batter, preventing a dense texture.
1 tbsp vanilla extract
This boosts the overall flavor with a warm, sweet aroma. Don’t skimp, vanilla rounds out the entire dessert.
1 cup all-purpose flour
Provides structure. Just the right amount keeps the blondies soft, not cakey.
¼ tsp baking powder
Adds a subtle lift to keep the blondies from being too dense.
½ tsp salt
Balances the sweetness and enhances the flavors. A small amount makes a big difference.
1 cup white chocolate chips
Sweet and creamy, white chocolate melts into the batter for rich pockets of flavor.
4 tbsp rainbow sprinkles
Adds fun color, texture, and whimsy! Use “jimmies” style sprinkles, they melt better into batter without bleeding.

For the Frosting:

¼ cup softened unsalted butter
Acts as a creamy base for the frosting, rich and smooth.
2 tbsp softened cream cheese
Adds a tangy balance to the sweetness and gives the frosting structure.
1 ½ cups powdered sugar
Sweetens and thickens the frosting to a smooth, spreadable consistency.
¼ tsp salt
Cuts the sweetness and enhances flavor, yes, even in frosting!
1 tsp vanilla bean paste
Brings a stronger, richer vanilla flavor with gorgeous specks. Vanilla extract works just as well if that’s what you have.
1 tbsp milk
Thins the frosting slightly for that perfect creamy spreadability.

Step-by-Step Instructions

1. Prep Your Pan & Oven

Preheat your oven to 350°F (175°C). Line a 9x9 baking pan with parchment paper, letting it hang over the sides for easy lifting later.

2. Melt the Butter

In a microwave-safe bowl or saucepan, melt ½ cup unsalted butter. Let it cool slightly—this helps prevent scrambled eggs when mixing later.

3. Mix Wet Ingredients

In a mixing bowl, stir together the melted butter and 1 cup brown sugar until smooth. Add the 2 eggs and 1 tbsp vanilla extract. Mix until just combined, don’t overmix!

4. Add Dry Ingredients

Add 1 cup all-purpose flour, ¼ tsp baking powder, and ½ tsp salt to the bowl. Use a spatula to gently fold everything together just until you don’t see any dry flour. Overmixing can lead to dense blondies.

5. Add the Fun!

Fold in 1 cup white chocolate chips and 4 tbsp rainbow sprinkles. Stir until evenly distributed.

6. Bake

Pour the batter into your prepared pan and smooth out the top. Bake for 30–35 minutes, or until the edges are golden and a toothpick inserted near the center comes out with just a few moist crumbs.

Pro Tip: Ovens vary! Start checking at 28–30 minutes if your oven runs hot.

7. Cool

Let the blondies cool in the pan for at least 20–30 minutes before frosting. You can speed this up by placing the pan on a wire rack.

🎂 Make the Frosting

1. Cream the Butter & Cream Cheese

In a stand mixer (or with a hand mixer), beat ¼ cup softened butter and 2 tbsp cream cheese on medium-high until creamy and smooth, about 1 minute.

2. Add Sugar & Flavor

Add 1 ½ cups powdered sugar, ¼ tsp salt, 1 tsp vanilla bean paste (or extract), and 1 tbsp milk. Beat for 2–4 minutes until fluffy and creamy.

Texture Tip: If your frosting is too thick, add a splash more milk. Too thin? Add more powdered sugar.

3. Frost & Decorate

Once blondies are cool, spread frosting evenly over the top. Add extra sprinkles for that irresistible funfetti finish!

Storage & Make-Ahead Tips

Room Temperature: Store frosted blondies in an airtight container for up to 2 days.
Fridge: Lasts up to 5 days chilled. Let them sit at room temp for 10 minutes before eating.
Freezing: Freeze unfrosted blondies (whole or sliced) wrapped tightly for up to 2 months. Thaw overnight in the fridge and frost before serving.

Troubleshooting & Tips

Problem	Solution
Blondies too dry?	Don’t overbake! Pull them when the edges are set but the center still looks slightly soft.
Blondies too dense?	Make sure not to overmix once you add flour. Gentle folding is key.
Frosting too runny?	Chill it for 10–15 minutes or add more powdered sugar.
Sprinkles bleeding color?	Use “jimmies” (long, rod-shaped) sprinkles, not nonpareils.

Customize It!

Gluten-Free? Swap flour for a 1:1 gluten-free baking blend.
Extra Flavor? Try browning the butter before mixing for a nuttier, deeper flavor.
Make it a Cake? Double the recipe and bake in a 9x13 pan for ~40 minutes.

I love making and eating birthday cake blondies because they remind me of the joy and excitement of celebrating with friends and family, but without the pressure of a complicated cake. If you want other birthday-inspired desserts, you may like Funfetti Cookies or Sugar Cookie Bars, too!

I may be a little biased because I LOVE birthday cake-flavored everything, but give these a try and let me know in the comments below how you like them!