Lightly coat an 8x8x2-inch baking pan with cooking spray; set aside. In a small bowl combine the 3/4 cup flour and the 3 tablespoons granulated sugar. Using a pastry blender or 2 knives, cut in butter until mixture resembles coarse crumbs. Press mixture into the bottom of the prepared pan. Bake in a 350 degree F oven for 15 minutes.
Meanwhile, for filling, in a small mixing bowl combine the egg and egg white. Beat with an electric mixer on medium speed until frothy. Add the 2/3 cup granulated sugar, the 2 tablespoons flour, the lemon juice, water, and baking powder. Beat on medium speed for 3 minutes or until slightly thickened. Stir in lemon peel. Pour filling over hot crust.
Bake for 20 to 25 minutes more or until edges are light brown and center is set. Cool on a wire rack. Cut into 9 squares; cut each square diagonally to make a triangle. If desired, sprinkle with sifted powdered sugar. Makes 18 triangles.