• 1 cup boiling water
• 3/4 cup unsweetened natural cocoa powder (not Dutch process)
• 1 3/4 cups all-purpose unbleached white flour
• 1 1/2 teaspoon baking soda
• 1/4 teaspoon salt
• 2 cups granulated sugar
• 10 tablespoons unsalted butter, at room temperature
• 3 large eggs
• 1 teaspoon pure vanilla extract
• 1 1/4 cups buttermilk

For the frosting:

• 3 3/4 cups powdered sugar
• 1 cup unsweetened natural cocoa powder (not Dutch process)
• 8 tablespoons unsalted butter, at room temperature
• 1 teaspoon pure vanilla extract
• About 1 cup whipping cream

• 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 until smooth. Let cool completely.
• In a bowl, sift together the flour, baking soda and salt.
• In a large bowl, using a handheld mixer on medium-high speed, beat together the sugar and butter 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 alternately with the buttermilk in 2 additions, beginning and ending with the flour mixture and stopping to scrape down the bowl as needed, beating until smooth. 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 to wire racks and let cool in the pans 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 off the parchment paper. Turn each cake right side up and let cool completely.
• To make the frosting, in a bowl, sift together the powdered sugar and cocoa. Using the mixer on low speed, mix in the butter until it is crumbly. Mix in the vanilla, and then gradually mix 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, then the sides, with the remaining frosting. Slice the cake into thick wedges and serve.