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 OREO O's® cereal and sprinkles.
Spread half of the batter evenly into prepared pan. Line OREO® cookies in an even layer over the batter.
Add remaining batter by large spoonfuls over the cookies and use a spatula or knife to evenly spread.
Bake on 350F (177C) for 35-40 minutes -- until edges just begin to turn golden brown.
Allow to cool before cutting and serving.