How to make it To prepare cake: Preheat oven to 375 degrees F. Coat two 9-inch round cake pans with cooking spray. Whisk all-purpose flour, whole-wheat flour, cocoa, baking powder and salt in a medium bowl. Beat sugar and butter in a large bowl with an electric mixer on medium speed until crumbly, about 2 minutes. Beat in egg yolks one at a time, then vanilla and food coloring (if using) until smooth. Beat in half the buttermilk on low speed until smooth, then half the flour mixture. Scrape down the sides of the bowl, then beat in the remaining buttermilk. Beat in the remaining flour mixture just until combined. Clean and dry beaters. Beat egg whites in a clean medium bowl at high speed until soft peaks form. With a rubber spatula, gently fold the whites into the batter until just incorporated, using long, even strokes. Divide the batter between the prepared pans, spreading to the edges. Gently rap the pans against the counter once or twice to settle the batter. Bake until a toothpick inserted into the center comes out clean, 15 to 20 minutes. Cool on a wire rack for 10 minutes, then invert the layers onto the rack, remove the pans and let cool to room temperature, about 45 minutes more. To prepare frosting & assemble cake: Beat cream cheese, confectioners' sugar and vanilla in a large bowl with an electric mixer on medium speed until smooth. Place one cake layer top-side down on a serving plate; cover with half the frosting, spreading just to the edges. Set the second layer on top, top-side down. Spread the remaining frosting on the top only. Sprinkle with grated chocolate, if desired. Ingredient Note If you prefer not to use food dye, you can omit it completely: just add 4 more teaspoons of buttermilk to the batter (your cake will, however, be more brown than red). Natural food dyes, while less vibrant than conventional dyes, are chemical-, lactose- and gluten-free, but they vary widely in availability. Check for them at your local natural-foods store. Cake-Baking Tips When using cake pans, they must be greased and floured to create a thin layer of protection against the oven's heat. For greater convenience, use a cooking spray that has flour in the mix, such as Pam for Baking, Baker's Joy or Crisco No-Stick Flour Spray. Whole-wheat pastry flour has less gluten-forming potential than regular whole-wheat flour, making it a better choice for tender baked goods. To properly measure flour when baking, use a spoon to lightly scoop flour from its container into a measuring cup. Once in the measuring cup, use a knife or other straight edge to level the flour with the top of the cup. If the measuring cup is dipped directly into the container, a common mistake, the flour will be packed into the cup and result in extra flour being added to the recipe, yielding tough, dense baked goods. Room-temperature butter for a batter is one of the biggest culinary missteps. In fact, butter must be below 68 degrees F to trap air molecules and build structure. Otherwise, the fat will be liquefied and the cake will be flat. To get "cool" butter: Cut refrigerated butter into chunks and let them sit in a bowl for 5 minutes before beating. Eggs must be at room temperature for the proteins to unwind enough to support the cake's crumb. Either set the eggs out on the counter for 15 minutes or submerge them in their shells in a bowl of lukewarm (not hot) water for 5 minutes.
