Preheat the oven to 350F (180C) and line cookie sheets with parchment paper or silicone baking mats.
In a medium-sized bowl, whisk together the flour, cocoa powder, corn starch, baking soda & salt. Set aside
In a large bowl, beat together the butter, brown sugar and granulated sugar until combined.
Beat the egg and vanilla extract into the butter mixture.
With the mixer on a low speed, beat the flour mixture into the butter mixture about 1/2 at a time until combined.
Turn off the mixer and stir in the chocolate chips and M&Ms. I typically reserve a few tablespoons of chocolate chips and M&Ms for dotting on the tops of the cookies.
Scoop the dough into balls, with each dough about 1 to 1.5 tablespoons of dough each. Place dough balls about 1.5-2 inches (4-5 cm) apart on the lined cookie sheets
Bake cookies 1 sheet at a time in the middle of the preheated oven for about 9-11 minutes. Take the cookies out of the oven when the tops look just set.
Cool cookies on the cookie sheets for at least 10 minutes, then transfer to a cooling rack. Optionally, place a few extra chocolate chips or M&Ms on top of each cookie.