Frosted Funfetti Blondies

5 from 1 vote

Frosted Funfetti Blondies are a birthday cake blondie recipe that's soft and chewy with buttery vanilla flavor, colorful sprinkles, and gooey marshmallow pockets. Brown sugar creates a rich, chewy texture while vanilla, butter extract, and rainbow jimmies give every bite that classic birthday cake flavor before a layer of creamy frosting finishes them off.

Prep Time15

Cook Time30

Cooling Time2

Total Time2 45

Course: Dessert, Kids

Cuisine: American

Servings: large blondies

Calories: 616kcal

Author: Jami Powell

Equipment

8×8-inch metal baking pan
Parchment paper
Large Mixing Bowl
Medium mixing bowl
Whisk
Rubber spatula
Wire cooling rack
Offset spatula or butter knife (for spreading the frosting)

Ingredients

Blondies

¾ cup unsalted butter melted and slightly cooled
1¼ cups packed light brown sugar
1 large egg room temperature
1 large egg yolk room temperature
1 tablespoon pure vanilla extract or 1 teaspoon vanilla extract plus ½ teaspoon butter extract
1¾ cups all-purpose flour
1 teaspoon baking powder
½ teaspoon salt
½ cup rainbow jimmies plus more for topping
¾ cup mini marshmallows tossed with 1 teaspoon flour

Vanilla Buttercream

½ cup unsalted butter softened
2 cups powdered sugar
2 tablespoons heavy cream or whole milk
1 teaspoon vanilla extract
¼ teaspoon butter extract optional
pinch salt
rainbow jimmies for decorating

Instructions

Preheat the oven to 350°F (177°C). Line an 8×8-inch metal baking pan with parchment paper, leaving enough overhang to lift the blondies out after baking.
Whisk the melted butter and brown sugar together until smooth and glossy, about 1 minute.
Add the egg, egg yolk, vanilla extract, and butter extract (if using). Whisk until completely combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Fold the dry ingredients into the wet ingredients just until combined. Do not overmix.
Fold in the rainbow jimmies, then gently fold in the floured mini marshmallows. Spread the thick batter evenly into the prepared pan.
Bake for 28 to 32 minutes, or until the edges are lightly golden and set while the center still looks slightly soft. The top should have a shiny, crackly finish, and a toothpick inserted near the center should come out with a few moist crumbs but no wet batter.
Let the blondies cool completely in the pan on a wire rack.

Make the Icing

While the blondies cool, beat the softened butter until creamy.
Gradually mix in the powdered sugar, then beat in the heavy cream, vanilla extract, butter extract (if using), and salt until light and fluffy. If needed, adjust the consistency with a little more cream or powdered sugar.
Spread the buttercream evenly over the blondies using an offset spatula or the back of a spoon. Immediately sprinkle the top with additional rainbow jimmies.
Allow the frosting to stand at room temperature for at least 20 to 30 minutes before slicing.
For clean, bakery-style squares, refrigerate the frosted blondies for 30 to 45 minutes before slicing. Run a sharp knife under hot water, wipe it dry, and slice in one smooth motion, cleaning the knife between cuts as needed.

Video

Notes

Cooking Notes

These blondies should still look slightly soft in the center when you remove them from the oven. They continue setting as they cool, creating the perfect chewy texture.

Tips for Best Results

Use rainbow jimmies instead of nonpareils to keep the colors bright.
Mix the batter only until the flour disappears for the softest blondies.
Toss the marshmallows with a little flour so they stay evenly distributed.
Wait until the blondies are completely cool before adding the frosting.

How to Know They're Ready

The edges should be lightly golden and set while the center still looks slightly soft. A toothpick inserted near the center should come out with a few moist crumbs, not wet batter.

Nutrition