Line cookie sheet with parchment paper, set aside.
In the bowl of a stand mixer, cream butter and sugars until smooth.
Add in egg and vanilla, beat until thoroughly combined.
In a separate medium mixing bowl whisk together flour, cornstarch, baking soda and salt.
With the mixer on low, slowly add the dry ingredients to the wet ingredients.
Using a rubber spatula fold in mini chocolate chips and M&M’s.
Using a cookie scoop, form balls about 2 tablespoons in size.
Place cookies on prepared cookie sheet and repeat until all the dough has been used.
Cover cookie sheet with plastic wrap.
Place the cookie sheet in the freezer for 1 hour.
Preheat oven to 350 degrees Fahrenheit.
Remove cookies from freezer. Space the cookies evenly on the cookie sheet. Bake cookies for 12 minutes, cookie should appear slightly under baked.
Press extra M&M’s into the cookies. Allow cookies to cool on cookie sheet for 5 minutes, transfer to a cooling rack.