Whip butter with an electric mixer until fluffy. Stir in the confectioners' sugar, sugar, cornstarch, flour, salt add vanilla last. Beat on low for one minute, then on high for 3 to 4 minutes.

Mix in M&M's with spoon or spatula.

Press dough in a 9x13 inch pan.

Bake at 350 degrees for 20 to 25 minutes slightly golden.

When cool cut into squares. Enjoy!