Preheat oven to 350F. Mix thoroughly flour, butter and powdered sugar. Press in ungreased 8x8x2 pan building up a 1/2 inch edge.

Bake for 20 minutes.

Beat remaining ingredients about 3 minutes or until light and fluffy. Pour over hot crust and continue baking for 25 minutes longer or just until no imprint remains when touched lightly in center. Cool and cut into squares. Dust with Powdered Sugar, if desired.