Preheat oven to 375 degrees. Line a 9 x 13-inch baking pan with parchment paper.
In a mixing bowl whisk together flour, cornstarch, baking powder, baking soda and salt, set aside.
In the bowl of an electric stand mixer fitted with paddle attachment, beat together butter and sugar on low speed until it starts to come together then increase to medium speed and whip until creamy, about 3 to 4 minutes. Reduce speed and add egg, egg yolk and vanilla.
With mixer on low speed, slowly add dry ingredients, beat just until combined. Stir in M&M's (Reserve about 1/4 cup of the M&M's and lightly press into the top of the cookie bars after baking so they show through more).
Press cookie dough into prepared baking pan. Bake for 18-20 minutes until edges are lightly golden. Allow to cool completely before cutting into bars.