Preheat the oven to 180ºC . Butter three 23 cm round cake pans. Whisk together the cocoa and the boiling water in a bowl, making a smooth paste. Set aside to cool. Sift together the cake flour, all-purpose flour, baking powder and baking soda. Put the butter and brown sugar in the bowl of an electric mixer. With the paddle attachment, cream them on medium-high speed for 2 minutes, until light and fluffy, Add the eggs, 1 at a time, beating well after each addition. Decrease speed to low, and stir in the dry ingredients alternate with the buttermilk. Stir in the reserved cocoa paste and the vanilla. Divide the batter evenly among the 3 pans. Bake for about 25 minutes, until a cake tester inserted in the middle comes out clean. When the cakes are cool, unmold them by running a knife around inside edge of each pan and inverting onto racks. Fill and frost with Bittersweet Chocolate Frosting, making a 3-layer cake. Serves 10 to 12.
*BITTERSWEET CHOCOLATE FROSTING*
Melt the chocolates and the butter over a pot of simmering water. Let the mixture cool to lukewarm. Sift together the powdered sugar and salt into a large bowl. Combine the vanilla and the milk. Whisk the milk into the powdered sugar. Add the melted chocolate mixture and stir until smooth.