Line a baking sheet with parchment paper and then set it aside.
In a stand up mixer, cream the sugars and the butter together well. Then, add in the egg and vanilla. Mix until combined.
In a large bowl, mix together the sifted all-purpose flour, baking soda, cornstarch and salt.
Slowly add in the dry ingredients to the wet ingredients. Mix until combined.
Fold in the M&Ms and the chocolate chips.
Scoop out about 2 tablespoons worth of cookie dough and roll it into a ball. Place on the parchment lined baking sheet. Continue this process until all the dough has been used up.
Freeze the cookie dough balls for one hour or overnight. To do this, you can wrap the baking sheet with plastic wrap and place in freezer. Or place cookie dough balls in a freezer bag.
Preheat oven to 350 degrees.
Remove cookie dough balls from freezer and bake 10-12 minutes.
Let cool on a wire rack for 10 minutes.
Store in an airtight container for up to a week and enjoy!