Sift together the flour, baking soda, salt, and cornstarch and set aside.
Using an electric mixer, cream together the soft butter and shortening
Mix in the granulated sugar and brown sugar until very creamy.
Add the eggs one at a time.
Mix in the pure vanilla.
Slowly add the dry ingredients which you sifted JUST until blended. Do not over mix.
Mix by hand 2 bag of Mega M & M's into the dough.
Use an ice cream scoop and drop cookies onto a baking sheet that has been lined with parchment paper.
BAKING TIP: Place the pan of cookie dough in the freezer at least 20 minutes before baking.
Bake 375 degrees for 24 minutes if using a 1/3 cup ice cream scoop.
Bake 15 minutes if using a regular size 1/4 cup ice cream scoop.