Mix dry ingredients together: In a large bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder and salt. Set aside.
Cut the butter into squares and using the paddle attachment of a stand mixer, cream the butter with the sugars for 2 minutes or until light and fluffy.
Then add in the egg, egg yolk and vanilla extract. Blend until mixed. If any of the butter gets stuck on the paddle attachment use a rubber spatula to knock it off.
Gradually add in the flour mixture until ingredients are incorporated.
Add in the chocolate chips and mix just until the chocolate chips are incorporated. Turn off the machine. Add in the M&Ms and stir in using a rubber spatula.
Scoop cookie dough onto baking sheet. (This recipe makes 6 LARGE cookies). Place extra M&M’s on the top of the cookies if desired.
Bake for 10-12 minutes, or until cookies are lightly browned on the top. Remove from oven and let the cookies rest on the cookie sheet for 15 minutes then transfer to a cooling rack and enjoy!
M&M creates seasonal M&Ms so you can use this recipe to make Easter cookies with the pastel color M&Ms, Christmas cookies with red & green seasonal M&Ms, Halloween M&Ms, etc. If you just want a good ol' M&M cookie, I recommend using the mini M&Ms!