Scottish Shortbread

1/2 cup butter, softened
1/2 generous cup sugar
1 scant cup cornstarch
1/2 cup margarine, softened
1 2/3 cups all-purpose flour

Blend well with hands. Press into a 13 x 9-inch glass baking dish. With fork, poke holes everywhere. Every inch must be perforated several times with holes that go to the bottom of the pan. Bake at 325 degrees F for 25 to 35 minutes, until golden brown. Let cool 5 to 10 minutes before slicing into bars.