Preheat oven to 350. Sift flour and powdered sugar into a 9 x 9 baking dish. Cream in the butter thoroughly. Pat out this crust to cover the bottom of the dish. Bake at 350 for 20 minutes.
Beat remaining ingredients together for 3 minutes (until light and fluffy). Pour over hot crust. Bake another 25 minutes or until no imprint remains when touched in center. Cool and cut into squares. Makes 16 cookies.
To make this gluten free, substitute your favorite wheat free flour. I recently made this with Brown Rice Flour and it was very well received. To make this dairy free choose a non-dairy margarine. I used Flieschman's unsalted and it really worked well.