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Preheat the oven to 350u00b0F. Butter two 9-by-2-inch round cake pans; line the bottoms with parchment paper. Butter parchment, and dust with flour, tapping out excess; set aside. Into a medium bowl, sift together flours, baking powder, and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, 3 to 4 minutes, scraping down the sides of the bowl as needed. Beat in eggs, one at a time, then beat in vanilla. With the mixer on low speed, add the flour mixture in three parts, alternating with the milk and beginning and ending with the flour; beat until combined after each addition. Divide the batter between the prepared pans, and smooth with an offset spatula. Bake, rotating the pans halfway through, until cakes are golden brown and a cake tester inserted in the centers comes out clean, 30 to 35 minutes. Transfer pans to a wire rack to cool 20 minutes. Invert cakes onto the rack; peel off the parchment. Reinvert cakes and let them cool completely, top sides up. Using a serrated knife, trim the tops of the cakes to make level. Place one layer on a cake plate, and spread top with 3/4 cup Dark Chocolate Frosting. Place the other cake layer on top. Spread the entire cake with remaining frosting, swirling to coat in a decorative fashion. Cake can be kept in the refrigerator, covered with a cake dome, for up to 3 days. Let cake sit at room temperature for 20 minutes before serving.
