Preheat the oven to 350°F. Line a 9×13-inch sheet pan with parchment paper, leaving an overhang for easy removal.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and glossy.

Add the eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract.

Sift in the all-purpose flour, cocoa powder, baking powder, and salt. Fold gently until just combined, taking care not to overmix.

Fold in the semi-sweet chocolate chunks, rainbow sprinkles, and chopped birthday cake-flavored cookies until evenly distributed throughout the batter.

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.

Allow brownies to cool completely in the pan placed on a wire rack.

Beat the softened butter with an electric mixer until creamy.

Gradually add the sifted powdered sugar, milk, vanilla extract, and a pinch of salt, continuing to beat until the frosting is light and fluffy.

Spread the prepared frosting evenly over the cooled brownies.

Decorate generously with extra rainbow sprinkles.

Cut into squares and serve.