1. Preheat oven to 350 degrees.

2. In a large bowl, mix the flour, baking soda, salt, and cornstarch together and then set aside.

3. Using a hand or stand mixer, cream the butter and sugars until thoroughly combined.

4. Add the egg, egg yolk, and vanilla to the sugar mixture and mix well.

5. While stirring, slowly add in the flour mixture until a thick dough forms. Fold in the M&Ms, but do not over mix as the M&Ms may crack.

6. Chill dough for 30 minutes or overnight (dough can be stored covered for up to five days) in the refrigerator.

7. Drop dough two inches apart onto an ungreased baking sheet and press a few M&Ms on top of each cookie.

8. Bake cookies for 8-10 minutes or until edges are browned just slightly. Cookies may appear to be undercooked but will harden as they cool.

9. Leave cookies on cookie sheet, until cool and remove to baking rack.