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Preheat oven to 350°.
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Spray an 8-inch square baking pan with non-stick cooking spray.
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In small saucepan, melt margarine and set aside to cool.
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In mixing bowl, stir together flour, cocoa, baking powder and salt.
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In separate bowl, beat egg and egg whites with an electric mixer.
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Whisk sour cream and vanilla into cooled margarine.
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Beat in the sugar.
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Thoroughly beat in egg mixture; pour over dry ingredients and beat well.
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Pour batter into pan.
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Bake until knife inserted in center is fairly clean, 20 to 25 minutes.
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Cool before cutting.