Preheat the oven to 350?F. Use 1 tablespoon butter to grease three 9-inch cake pans. Line the bottom of each pan with a parchment paper round.
Combine the cocoa and chocolate in a medium bowl. Pour the boiling water over the chocolate mixture and whisk until smooth; reserve. Sift together the flours, baking soda, and salt into a medium bowl; reserve.
Place the 3/4 pound butter in a large bowl and beat with an electric mixer on medium-high speed until creamy, about 1 minute. Add the dark brown and granulated sugars and beat on high until light and fluffy, about 3 minutes. Stop the mixer and scrape down the bowl with a rubber spatula. Add the eggs, one at a time, beating 30 seconds on medium-high after each addition. Reduce the speed to medium and add the sour cream and vanilla. Beat until combined. Stop the mixer and scrape down the bowl. With the mixer on low, add about one-third of the flour mixture, followed by about one-half of the chocolate mixture. Repeat, ending with the flour mixture and beating until just combined. Stop the mixer and scrape down the sides of the bowl. Stir the batter gently to thoroughly combine. Divide the batter evenly among the cake pans and smooth with a spatula. Bake until a skewer inserted in the center of each cake layer comes out clean, about 25 minutes. Cool the cakes in their pans on a wire rack for about 15 minutes. Invert the cakes, one at a time, onto a large plate, peel off the parchment, and invert onto the rack.
Cool completely before icing between layers, on sides, and on top with Vanilla Buttercream Icing. Serve at room temperature. (Any buttercream will not fare well in too warm conditions.)
Makes one 9-inch 3-layer cake, 16 servings