Spray an 8x8x2-inch baking pan with nonstick spray; set pan aside.
In a small mixing bowl combine 3/4 cup flour and 3 tablespoons granulated sugar. Cut in margarine until crumbly. Pat mixture onto the bottom of the prepared pan. Bake in a 350 degree F oven for 15 minutes.
Meanwhile, in the same bowl combine egg and egg white. Beat with an electric mixer on medium speed until frothy. Add 2/3 cup granulated sugar, 2 tablespoons flour, lemon peel, lemon juice, water, and baking powder. Beat on medium speed for 3 minutes or until slightly thickened. Pour mixture over baked layer in pan. Bake for 20 to 25 minutes more or until edges are light brown and center is set. Cool in pan on a wire rack.
Sift powdered sugar over top. Cut into bars. Store in refrigerator. Makes 16 bars.