Preheat oven to 300 F.
Cream the butter. Stir in the sugar until well blended. Sift in the flour and cornstarch. Add the almond or vanilla extract, if you wish. Stir until the mixture just begins to come together. Take care not to overwork the dough because this will make the cookie tough and shortbread should be delicate.
Pat the dough into a circle about 1/2 inch thick on a highly oiled cookie sheet. Punch a bunch of holes in the dough with a fork. Bake for 50 to 60 minutes, until just beginning to turn golden, but not brown.
Cut into wedges while still warm.
From "Sundays at Moosewood Restaurant"