Preheat oven to 325F degrees. Mix the flour, baking soda, and cornstarch in a bowl and set aside. With your mixer, cream the butter and sugars together. Add the egg, egg yolk, and vanilla and mix well. Gradually add the flour mixture and stir until a dough forms. Fold in the M&Ms and sprinkles. Do not over mix the dough at any point in this process. Chill dough for at least 30 minutes in the refrigerator. Drop by tablespoonful's onto ungreased baking sheet. Press a few more M&Ms onto the tops for decoration. Bake for 10-12 minutes or until edges are slightly browned. The centers should appear very soft and puffy. Let cool.