Preheat oven to 350F (177C) and prepare a 9x9 pan by lining with parchment paper (or lightly grease and flour).
Combine melted butter and sugars in a large bowl, stir well.
1 cup butter, melted, 1 ¼ cup sugar, ½ cup light brown sugar tightly packed
Add eggs, and egg yolk one at a time, stirring well after each addition.
2 eggs + 1 egg yolk room temperature preferred
Stir in vanilla extract.
2 teaspoon vanilla extract
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
2 ½ cups all purpose flour 285g, 2 teaspoons cornstarch, ½ teaspoon baking powder, 1 teaspoon salt
Gradually stir dry ingredients into wet until completely combined. Stir in chocolate chips and then sprinkles.
½ cup mini semisweet chocolate chips, ¼ cup colored sprinkles
Spread half of the batter evenly into prepared pan. Firmly press 16 Oreo cookies in an even layer over the batter.
16 + 5 Oreo cookies
Drop remaining batter by large spoonfuls over the cookies and use a spatula or knife to evenly spread over the cookies.
Gently break 5 remaining Oreos into pieces and press firmly into the top of the batter, if desired.
Bake on 350F (177C) for 35-40 minutes -- until edges just begin to turn golden brown and a toothpick inserted in the center comes out clean or with few moist crumbs.
Allow to cool before cutting and serving (don't cool too long, though -- these are amazing when served still slightly warm!)