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