01 -
Preheat the oven to 350°F. Line a 9x13-inch sheet pan with parchment paper, leaving an overhang for easy removal.
02 -
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and glossy.
03 -
Add the eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract.
04 -
Sift in the all-purpose flour, cocoa powder, baking powder, and salt. Fold gently until just combined, taking care not to overmix.
05 -
Fold in the semi-sweet chocolate chunks, rainbow sprinkles, and chopped birthday cake-flavored cookies until evenly distributed throughout the batter.
06 -
Spread the batter evenly in the prepared sheet pan. Bake for 23 to 25 minutes, or until the center is just set and a toothpick inserted yields moist crumbs. Do not overbake.
07 -
Allow brownies to cool completely in the pan placed on a wire rack.
08 -
Beat the softened butter with an electric mixer until creamy. Gradually add sifted powdered sugar, milk, vanilla extract, and a pinch of salt, continuing to beat until the frosting is light and fluffy.
09 -
Spread the prepared frosting evenly over the cooled brownies. Decorate generously with extra rainbow sprinkles.
10 -
Cut into squares and serve.