Preheat oven to 350-degrees. Line the bottom of a pie dish or 8-inch circle cake pan with parchment paper, spray the sides and bottom of the dish with baking spray and set aside.

In the bowl of a stand mixer, affixed with the paddle attachment, add the butter and both sugars. Mix on medium-high speed until the mixture is light and fluffy. Add the egg and vanilla extract. Mix until fully combined.

Change the mixer to low speed and add the flour, salt, cornstarch, baking soda and baking powder. Mis until just combined. Remove the bowl from the mixer and use a rubber scraper to ensure all the ingredients are fully incorporated. Fold in the chocolate chips and M&M's.

Pour dough into prepared dish and evenly distribute the dough into the pan (wet your fingers slightly to push the dough evenly into the pan). Bake the cookie in a 350-degree oven for 20-25 minutes or until the edges are a light brown and the middle doesn't jiggle. Do not over bake though. Remove from the oven and set on a wire rack to cool completely.