Preheat oven to 350 degrees. Butter two 8-by-2-inch round cake pans. Line with parchment paper, and butter again. Dust with flour, and tap out the excess; set aside. In a medium bowl, sift together cake flour, cocoa, and baking powder; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the butter and sugar, and beat on medium speed until light and fluffy, about 15 minutes. Add eggs, one at a time, beating well after each addition. Add vanilla, salt, food coloring, and water; beat, scraping down the sides of the bowl, until well combined. Add reserved dry ingredients in additions, alternating with the buttermilk; beat on low speed until well combined.
In a small bowl, stir together baking soda and vinegar; add to batter. Beat on medium speed for 10 seconds. Be sure not to overbeat. Divide batter between prepared pans, and bake until a cake tester inserted into the center comes out clean, 25 to 30 minutes. Remove pans from oven, and transfer to a wire rack to cool, about 15 minutes. Turn cakes out onto rack; let stand until completely cool.
Using a serrated knife, trim tops of cakes so surfaces are level. Place a layer on a cake stand or cardboard round, and spread about 1/3 of the frosting over top. Place the second layer on top, and spread with the remaining frosting over the tops and sides of assembled cake. Transfer to refrigerator, and chill until ready to serve.