Preheat oven to 350 degrees.
Line cookie sheets with parchment paper.
Add flour, baking soda, and cornstarch in a medium bowl.
Mix to combine. Set aside.
Using your electric mixer add butter and both sugars.
Cream together until smooth and without lumps.
While mixer is on, slowly add the egg, egg yolk, and vanilla. Mix well.
Slowly add the flour mixture, stirring until completely mixed.
Stir in the M&Ms and sprinkle. Stir just to combine.
Transfer cookie dough to the refrigerator. Chill for 30 minutes.
Measure cookie dough with a tablespoon placing the dough on the prepared cookie sheet.
Bake at 350 degrees for 10-12 minutes.
Check that the cookie edges are slightly brown.
The cookie centers will be soft and puffy.
Allow the cookies to cool slightly on the cookie sheets before transferring them to a wire rack to cool completely.
Makes 24 cookies