Mix melted butter, brown sugar, and white sugar together
Add egg and vanilla extract to the mixture
In a separate bowl, combine flour, cornstarch, baking soda, and baking powder
Mix dry ingredients into wet ingredients
Fold in crushed oreos, graham crackers, marshmallow fluff, chocolate chips, and Hershey's bar pieces
Bake at 325°F for 9-10 minutes until golden brown around the edges (may take 12-14 minutes depending on cookie size)
Cool and serve
