In food processor bowl, combine flour, sugar, baking powder and salt. Drop in butter and pulse until mixture resembles coarse meal. Pour into a bowl and set aside. In a small bowl blend milk, lemon peel and one whole egg. Pour egg mixture into flour mixture, gently mix to form a soft ball of dough. On a floured surface, divide dough in half. Roll into 9 inch circles. Prick each circle with a fork. Brush generously with the remaining egg white, beaten. Sprinkle tops with additional sugar. Bake at 425 degrees for 10 to 12 minutes or until golden brown. Assemble shortcake using hot or cold shortbread.