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.
Add eggs, and egg yolk one at a time, stirring well after each addition.
Stir in vanilla extract.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually stir dry ingredients into wet until completely combined. Stir in chocolate chips and then sprinkles.
Spread half of the batter evenly into prepared pan. Firmly press 16 Oreo cookies in an even layer over the batter.
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!)