In a bowl, cream together the butter & sugar until light and fluffy. Add the flour; using your fingers, work together to a crumbly but moist dough. Place mixture in baking pan and press down with the back of a spoon until firm and smooth. Bake shortbread until cooked but not browned, about 12 minutes. Remove from oven, lift out of pan with parchment paper & cool. When cooled, break into pieces & place in a food processor. Pulse to create shortbread crumbs. Set aside.