Preheat oven to 350 degrees F. Line a 9x13 rectangular metal baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside.

In the bowl of a stand mixer, cream together the butter, brown sugar and granulated sugar with the paddle attachment until creamy and pale, about 1 minute. Add in the egg, followed by the egg yolk and vanilla, beating well after each addition. Lastly, add in the baking soda, cornstarch, salt, and flour and mix on low speed until incorporated. Add in the chocolate chips and M&M's and mix slowly to combine.

Press the dough into the prepared pan in an even layer. If you'd like, dot the top with additional M&M's to make it look extra pretty.

Bake for 25-30 minutes or until the top is golden brown and set. Cool completely before cutting into squares or bars.