Ingredients
3 sticks unsalted butter, room temperature, plus more for pans
1 cup boiling water
3/4 cup unsweetened Dutch-process cocoa powder
4 ounces bittersweet chocolate, chopped (3/4 cup)
3 1/2 cups all-purpose flour
1 teaspoon baking powder
3/4 teaspoon baking soda
1 1/2 teaspoons coarse salt
2 cups packed light-brown sugar
4 large eggs, room temperature
2 teaspoons pure vanilla extract
1 cup sour cream
6 1/4 cups Chocolate Swiss Meringue Buttercream Frosting
Preparation
Preheat oven to 325 degrees. Butter two 9-inch round cake pans. Line bottoms with parchment rounds; butter parchment. Stir together boiling water, cocoa, and chocolate in a bowl. Let cool 10 minutes. Whisk together flour, baking powder and soda, and salt in another bowl.
In a mixing bowl, beat butter with sugar on medium-high until fluffy, 2 to 3 minutes. Beat in eggs, one at a time, scraping down sides of bowl. Beat in vanilla, then chocolate mixture. Reduce speed to low; beat in flour mixture in two additions, alternating with sour cream, until just combined.
Divide batter evenly between prepared pans, smoothing tops with an offset spatula. Bake until a tester inserted in centers comes out with moist crumbs attached, 35 to 40 minutes.
Let cakes cool in pans on a wire rack 20 minutes. Turn out onto rack, top-sides up; let cool completely. With a serrated knife, trim tops of cakes so they’re level.
Line the edges of a cake stand or plate with strips of parchment. Place one cake layer on stand, trimmed-side up, and spread 1 1/2 cups buttercream evenly over top. Top with remaining cake layer, trimmed-side down.
Spread a thin layer of buttercream over top and sides. Refrigerate until firm, about 30 minutes (this ensures a crumb-free finish). Spread remaining frosting evenly over top and sides. Cake can be stored in refrigerator up to 1 day; bring to room temperature before serving.
