Preheat oven to 325F degrees.
Mix the flour, baking soda, salt and cornstarch in a bowl and set aside.
Melt the butter, then add it to a separate bowl along with the sugars; be sure to mix until there are no lumps.
Add the egg, egg yolk, and vanilla and mix well.
Gradually add the flour mixture and stir until a dough forms.
Fold in the mini M&Ms and chocolate chips.
Drop by tablespoonfuls onto ungreased baking sheet.
Press a few more M&Ms onto the tops for decoration.
Bake for 10-15 minutes or until edges are slightly browned.
Let cool completely and enjoy!
