In a medium-sized bowl whisk together the flour, cocoa powder, cornstarch, baking soda & salt.
In a separate large-sized bowl using an electric mixer beat together the melted butter, sugars and melted chocolate until evenly combined. Add in the egg and vanilla and continue beating until mixed.
Turn the mixer down to low speed and carefully beat in the flour mixture. Then turn off the mixer and stir in the M&Ms and chocolate chips. Cover the bowl with clingfilm and refrigerate for 2 hours or overnight.
When ready to bake, preheat the oven to 350F degrees and line 2 cookie sheets with parchment paper or silicone baking mats. Allow the cookie dough to warm up for about 15 minutes or a little longer if it was in the fridge overnight.
Form the dough into balls about 1.5 tablespoons in size and place 2 inches apart on the cookie sheet. Bake for 8-10 minutes, or until the tops look just set. After cooking, you can place a few extra M&Ms & chocolate chips on the top of each cookie.
*Cookies store well in an air-tight container at room temperature for about 4 days.
**Cookie dough balls can be frozen in a ziploc bag for about 2 months. Then the dough balls can be cooked from frozen. They'll need about 1 extra minute baking.