Sift flour and powdered sugar into bowl. Blend in butter with clean fingertips until well mixed. Pat evenly into the bottom of an 8x8 inch baking pan. Bake for 20 minutes at 350 degrees. Meanwhile, beat together the other ingredients. Pour over baked crust and return to oven for 20-25 minutes at same temperature.
Cool on rack. Cut into squares. Sprinkle with sifted, powdered sugar.