In a bowl, mix together flour, cornstarch, salt and baking soda.
In the bowl of a stand mixer cream together butter and sugars until light and fluffy (about 2-3 minutes).
With the mixer going very slow, add in the egg and the vanilla.
Mix until all ingredients are incorporated.
Scrape sides of the bowl if needed.
Now add dry ingredient mixture gradually until dough forms.
Stir in the M&Ms by hand.
Refrigerate dough for 30 minutes.
Preheat oven to 350 degrees F and line baking sheets with parchment paper.