Combine flour and powdered sugar in a small mixing bowl; stir well. Cut in butter with a pastry blender until mixture resembles coarse meal. Press firmly into a lightly greased 9- inch square baking pan. Bake at 350° for 15 minutes. Remove from oven, and set aside.
Beat egg yolks in a large mixing bowl; add remaining ingredients, stirring well.
Beat egg whites (at room temperature) in a medium mixing bowl until stiff peaks form. Fold into egg yolk mixture. Pour over reserved hot crust. Bake at 350° for 30 minutes or until browned. Cut into 1-inch squares while warm.