Heat oven to 350 degrees. Mix flour, butter, and powdered sugar. Press mixture into an ungreased square pan, building up 1/2" around the edges. Bakes for 15-20 minutes. While the crust is baking, beat the remaining ingredients together until light and fluffy, about 3 minutes. When the crust is done, pour filling over the hot crust. Bake until no indentation remains when touched lightly in the center, about 25 minutes. Cool and cut into squares.