In the bowl of a stand mixer, cream together the butter, cream cheese, brown sugar, and sugar until light and fluffy.
Mix in the egg and vanilla until combined.
Add the flour, cornstarch, baking soda, and salt and mix for about 1 minute, until cookie dough begins to form.
Stir in the M&Ms by hand.
Use a cookie scoop (mine held 1 Tablespoon of dough) and arrange the cookies on a parchment paper lined baking sheet or plate.
Press down on the cookie to flatten, then chill the dough in the refrigerator for at least two hours.
After chilling, place the cookie dough balls on a baking sheet lined with parchment paper about two inches apart.
Bake the cookies at 350 degrees for 8-9 minutes. Do not over bake, the cookies will become firm as they cool.
Allow the cookies to rest on the baking sheet for 5 minutes and then transfer them to a baking sheet.