Line cookie sheet with parchment paper, set aside.
In the bowl of a stand mixer, cream butter and sugars until smooth.
Beat in egg, peppermint, and vanilla until thoroughly combined.
In a separate medium mixing bowl whisk together flour, cocoa powder, cornstarch, baking soda, and salt.
With the mixer on low, slowly add the dry ingredients to the wet ingredients until combined.
Using a rubber spatula fold in ¼ cup of M&M’s.
Using a cookie scoop , form balls about 1 tablespoon in size.
Press 3 extra M&M’s into the cookies.
Place cookies on a prepared cookie sheet and repeat until all the dough has been used.
Cover cookie sheet with plastic wrap and place the cookie sheet in the freezer for 1 hour.
Preheat the oven to 350 degrees Fahrenheit.
Remove cookies from the freezer.
Space the cookies evenly on the cookie sheet. Bake cookies for 10-12 minutes, cookie should appear slightly underbaked
Allow cookies to cool on a cookie sheet for 5 minutes, transfer to a cooling rack.