Preheat the oven to 325°F and grease a 9-inch square pan. Combine the flour and rice flour or cornstarch in a large bowl. Cut the butter into small pieces and toss it in the flour so that all the bits are coated. Rub the butter into the flour until the mixture looks like bread crumbs. Stir in all except 2 teaspoons of the sugar. Bring the mixture together with your hands until it begins to form a crumbly dough. Press this into the prepared pan. Smooth the surface and pierce it at regular intervals with the tines of a fork. Sprinkle the reserved sugar on top and bake for 25–35 minutes or until it is dry and light gold. In Scotland shortbread is often shaped in a wooden form, then baked on a baking sheet. In either case, it is cut into serving pieces while still warm.
2 cups all-purpose flour 1 tablespoon rice flour or cornstarch 2 sticks (8 ounces) cold, unsalted butter (do not use margarine) ½ cup superfine or granulated white sugar