Preheat oven to 350°F and line a baking sheet with parchment paper.
Cream together softened butter and sugars in a bowl.
Mix in vanilla and egg until smooth.
Gradually add dry ingredients: flour, cornstarch, baking soda, and salt; stir until combined.
Fold in chocolate chips and sprinkles.
Using a 1/4 cup scoop, form dough, flatten, place Oreo in center, then seal dough around it.
Arrange on baking sheet and bake for 10 minutes; cookies will be slightly gooey.
Immediately top with extra chocolate chips, cool for 5 minutes, then transfer to a rack.