In a medium-sized bowl, whisk together the melted butter, brown sugar and granulated sugar until combined.
Add the room temperature egg and vanilla extract and whisk until completely combined and smooth.
Add the flour, cornstarch, baking soda and salt and fold together until a dough forms.
Add the crushed Oreos and chocolate chips/chunks and fold until evenly incorporated throughout the dough. Cover and let chill for 30 minutes in the fridge.
Roll the dough into large balls (about 3 tablespoons) and place on a baking sheet 3-4 inches apart. They will spread a bit on the pan.
Bake the cookies for 14-15 minutes until the edges are set. Let cool and enjoy!