To make the cake, preheat an oven to 350°F. Lightly butter two 9-inch round cake pans. Line the bottom of each pan with a round of parchment paper. Dust the pans with flour, tapping out the excess.
In a small heatproof bowl, whisk together the boiling water and cocoa powder until smooth. Let cool completely.
In a bowl, sift together the flour, baking soda and salt. Set aside.
In a large bowl, using a handheld mixer, beat together the granulated sugar and butter on medium-high speed until the mixture is light in color and texture, about 3 minutes. Beat in the eggs, one at a time, then beat in the vanilla and the cooled cocoa mixture.
Reduce the speed to low and add the flour mixture in 3 additions, alternating with the buttermilk and beginning and ending with the flour mixture. Beat until smooth, stopping the mixer occasionally to scrape down the sides of the bowl. Divide the batter evenly between the prepared pans and smooth the tops.
Bake the cakes until they begin to pull away from the sides of the pans, 35 to 40 minutes. Transfer the pans to wire racks and let cool for 15 minutes. Run a knife around the inside of each pan to release the cake. Invert the pans onto the racks, lift off the pans and peel the parchment paper off the cakes. Turn the cakes right side up and let cool completely.
Meanwhile, make the chocolate frosting: In a bowl, sift together the confectioners sugar and cocoa powder. Using the mixer, beat in the butter on low speed until it is crumbly. Beat in the vanilla, then gradually beat in enough of the cream to make a spreadable frosting.
Place 1 cake layer, bottom side up, on a cake plate. Using an icing spatula, spread the top of the layer with a generous 1/2 cup of the frosting. Place the second layer, top side down, on top of the first layer. Frost the top and then the sides with the remaining frosting. Slice the cake into thick wedges and serve. Serves 10.
Variation: For chocolate cupcakes, line 24 muffin cups with paper liners. Spoon the batter into the prepared cups. Bake until the tops spring back when pressed in the center, 20 to 25 minutes. Let the cupcakes cool before frosting.
Adapted from Williams-Sonoma Comfort Food, by Rick Rodgers (Oxmoor House, 2009).
