In a large mixing bowl beat together the butter and cream cheese.
Add sugars, egg, and vanilla and beat until light and fluffy, 4-5 minutes.
In a separate bowl combine the flour, cornstarch, baking soda, and salt. Add to the creamed mixture and stir just until combined.
Fold in both types of chocolate chips and the m&m's (I like to reserve a few m&m's to place on top of the cookies right when they come out of the oven).
Spoon cookie dough, rolling gently, onto a parchment lined cookie sheet.
Cover with plastic wrap and refrigerate for 15 minutes.
Preheat oven to 350 degrees F.
Remove cookies from the fridge and bake for 8-10 minutes. (They may appear slightly soft when you take them out of the oven, but don't worry, they harden has they cool!)
Allow to cool on the baking sheet for 5 minutes, then move them to a wire cooling rack to cool completely.
The baked cookies and the unbaked cookie dough both freeze really well!