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Preheat the oven to 350°F (175°C) and grease and flour three 9-inch round pans.
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Sift together 2 1/2 cups of cake flour, 1 teaspoon of baking soda, and 1/2 teaspoon of salt and set aside.
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In a small saucepan, heat 1/2 cup of water and 4 ounces of German sweet chocolate until melted. Remove from heat and let cool.
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In a large bowl, cream 1 cup of softened butter and 2 cups of white sugar until light and fluffy.
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Beat in the 4 egg yolks one at a time, then add the melted chocolate mixture and 1 teaspoon of vanilla extract.
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Beat in the flour mixture alternately with 1 cup of buttermilk, making sure to mix only until incorporated.
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In a separate large glass or metal mixing bowl, beat 4 egg whites until stiff peaks form.
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Fold 1/3 of the egg whites into the batter, then quickly fold in the remaining egg whites until no streaks remain.
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Pour the batter evenly into the three 9-inch pans.
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Bake in the preheated oven for 30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
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Allow to cool for 10 minutes in the pan, then turn out onto a wire rack.