Preheat oven to 400 degrees.
In a large bowl cream together butter, brown sugar and granulated sugar for 3 minutes on medium speed.
Add in eggs and vanilla and mix until well blended.
Mix in flour, cornstarch, baking powder, baking soda and salt until almost incorporated.
Add in chocolate chips, M&Ms and Oreo bits and finish mixing until the flour is just incorporated.
Using a large cookie scoop* or 1/4 cup measuring cup, scoop up dough and place onto a baking sheet (six balls of dough per sheet).
Bake for 11-13 minutes or until lightly golden brown.
Add a few more chocolate chips, M&Ms and Oreo bits to the top of each cookie immediately after removing from the oven.
Let them cool for 5 minutes on the baking sheet then transfer to a cooling rack.