1 cup boiling water
3/4 cup unsweetened cocoa powder (not Dutch-process)
1/2 cup whole milk
1 teaspoon vanilla
2 cups all-purpose flour
1 1/4 teaspoons baking soda
1/2 teaspoon salt
2 sticks (1 cup) unsalted butter, softened
1 1/4 cups packed dark brown sugar
3/4 cup granulated sugar
4 large eggs
Brown sugar buttercream or chocolate sour cream frosting
Preheat oven to 350°F. Butter 3 (8- by 2-inch) round cake pans and line bottoms of each with rounds of wax or parchment paper. Butter paper and dust pans with flour, knocking out excess.
Whisk together boiling water and cocoa powder in a bowl until smooth, then whisk in milk and vanilla. 3Sift together flour, baking soda, and salt in another bowl.
Beat together butter and sugars in a large bowl with an electric mixer until pale and fluffy, then add eggs 1 at a time, beating well after each addition. Beat in flour and cocoa mixtures alternately in batches, beginning and ending with flour mixture (batter may look curdled).
Divide batter among pans, smoothing tops. Bake in upper and lower thirds of oven, switching position of pans halfway through baking, until a tester comes out clean and layers begin to pull away from sides of pans, 20 to 25 minutes total. Cool layers in pans on racks 10 minutes, then invert onto racks, remove wax paper, and cool completely.
Put 1 cake layer, rounded side up, on a cake plate and spread with about 1 cup buttercream. Top with another cake layer, rounded side up, and spread with another cup buttercream. Top with remaining cake layer and frost top and sides of cake with remaining buttercream