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Spray a 9x1-1/2-inch round baking pan with nonstick spray; set pan aside.
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In a medium mixing bowl combine flour, sugar, cocoa powder, and baking soda.
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Add water, cooking oil, vinegar, and vanilla.
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Stir until thoroughly combined.
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In a medium mixing bowl beat the egg whites with an electric mixer on high speed until stiff peaks form.
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Fold into batter.
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Pour batter into the prepared baking pan.
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Bake in a 350 degree F oven about 25 minutes or until wooden toothpick inserted near the center comes out clean.
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Cool cake in pan on wire rack for 10 minutes.
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Remove cake from pan and cool thoroughly on wire rack.
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Spread whipped desert topping over top and sides of cooled cake.
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Top with pie filling.