Cream the powdered sugar and the butter, and mix in the flour a little at a time until thoroughly blended. Spread in about 1/2 inch (1 cm) thickness on a cookie sheet, and prick all over with the tines of a fork. Bake at 300F (150C) for about 30 minutes, until light golden brown. Sprinkle with granulated sugar immediately after removing from oven and allow to cool for 10 minutes before cutting into bars or squares. Allow to cool completely before removing from pan.