Preheat the oven to 350 F/180 C and line a square 8x8 baking pan with parchment to line all four sides.
In a large bowl, whisk together the light brown sugar, granulated sugar, egg, egg yolk, and vanilla extract until fluffy, about 1 minute.
Stream the melted butter into the bowl while whisking constantly until completely combined.
Fold the flour, baking powder, salt, and rainbow sprinkles into the batter until just combined.
Spread all but about 2/3 cup of the cookie batter evenly into the baking pan with an offset spatula.
Making the Brownie Batter
Melt the butter and chocolate chips together until smooth.
In another medium bowl, whisk together the sugar, vanilla extract, and egg until fluffy, about 1 minute. Stream the melted butter and chocolate into the bowl, whisking constantly until completely combined.
Fold the flour, cocoa powder, and salt into the batter until combined.
Pour the batter on top of the cookie batter in the pan and spread it evenly with an offset spatula. Use a spoon to dollop the rest of the cookie batter on top of the brownie layer, then sprinkle with more rainbow sprinkles.
Bake the brownies for 25-30 minutes or until a toothpick inserted into the center comes out with just the tip smeared in chocolate. Let them cool completely on a wire rack before cutting, then enjoy!
Video
Notes
*Measure your dry ingredients properly. This is my #1 baking tip! Do not ever scoop a measuring cup into your flour as this always leads to using too much flour. Instead, use the spoon-level method. This means fluffing the flour first, then spooning it into your measuring cup/spoon. For the BEST results, use a kitchen scale!*Bake them in a metal pan. I highly recommend using a metal baking pan, not glass or ceramic, as they conduct heat differently and require a longer baking time.Use a sharp knife for clean cuts: For bakery-worthy slices, run a sharp knife under hot water and wipe it clean between cuts.