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Preheat oven to 350°F.
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Spray the bottom and sides of a 10-inch spring-form pan with nonstick cooking spray.
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Dust the pan with 1 tablespoon cocoa and tap out the excess.
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To prepare batter: In a medium bowl, sift together 1/3 cup plus 1 tablespoon cocoa, flour, baking power and baking soda.
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Set aside.
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In a large bowl, with an electric mixer, beat egg whites, brown sugar, yogurt and vanilla until blended, about 1 to 2 minutes on medium speed.
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Mix in dry ingredients by hand or on low mixer speed until just moistened.
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Do not over-mix.
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(This is not a smooth batter.).
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Pour batter into prepared pan and bake for about 35 minutes, or until a toothpick inserted in the center comes out clean.
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(If the pan is of a different size, the cooking time may vary.) Cool in the pan on a wire rack for 15 minutes.
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Then use a knife to loosen the sides of the cake from the pan.
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Remove the ring and cool completely on a rack.
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Serve the cake on the metal bottom of the pan.
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To prepare the topping: in a small bowl, combine the remaining tablespoon of cocoa with the powdered sugar.
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Place the mixture in a small fine sieve and dust the top of cake before serving.