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Preheat your oven to 350 degrees. Grease and lightly flour three 9-inch cake pans.
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Sift the flour and baking soda together and set aside.
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With an electric mixer, cream the butter and shortening with the sugar until fluffy, about 5 minutes. Add the egg yolks one at a time, beating well between each addition. With the mixer on medium speed, add the flour and buttermilk alternately, beginning and ending with buttermilk. Add the vanilla and coconut, and stir well to incorporate.
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In a separate bowl with clean beaters, whip the egg whites to stiff peaks. Gently fold the beaten egg whites into the batter, just until blended.
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Pour the batter into the prepared pans and bake for 25-35 minutes or until a toothpick inserted in the center of the cake comes out clean.