For the cake:
1 3⁄4 cups all-purpose flour
1 cup cocoa powder
1 1⁄2 teaspoons baking soda
1⁄4 teaspoon salt
3⁄4 cup unsalted butter, at room temperature
2⁄3 cup granulated sugar
2⁄3 cup light brown sugar, packed
2 extra-large eggs, at room temperature
2 teaspoons vanilla extract
1 cup buttermilk, at room temperature
1⁄2 cup sour cream, at room temperature
1 teaspoon espresso powder
For the buttercream frosting:
1 pound unsalted butter at room temperature
10 ounces bittersweet chocolate
8 ounces semisweet chocolate
1⁄2 cup egg white, at room temperature (from 3 extra-large eggs)
1 cup granulated sugar
1/2 teaspoon salt
2 teaspoons pure vanilla extract
2 teaspoons instant espresso powder, dissolved in 1 teaspoon water
1 pinch cream of tartar
2 tablespoons milk
2 tablespoons dark rum (optional)
For the cake:
1. Preheat the oven to 350°F Butter two 8-inch round cake pans. Line the bottoms with parchment paper, butter the paper, and dust the pans with flour, knocking out any excess. In a medium bowl, sift together the flour, cocoa, baking soda and salt.
2. In the bowl of an electric mixer, fitted with the paddle attachment, cream the butter and sugars on high speed until light, approximately 5 minutes. Add the eggs and vanilla and mix well. Combine the buttermilk, sour cream, and coffee. On low speed, add the flour mixture and the buttermilk mixture alternately in thirds, beginning with the buttermilk mixture and ending with the flour mixture. Mix the batter only until blended.
3. Divide the batter between the two pans and smooth the tops with a spatula. Bake on the middle rack of the oven for 25 to 30 minutes, or until a toothpick comes out clean. Cool for 10 minutes on a rack, remove from the pans, and allow to finish cooling.
For the frosting:
4. Chop the chocolates and melt in a bowl over simmering water until smooth. Allow to cool.
5. Mix the egg whites, sugar, cream of tartar and salt in the bowl of an electric mixer, fitted with a whisk. Heat the egg whites in the bowl over simmering water until they are warm to the touch, about 5 minutes. Whisk on high speed for 5 minutes, or until the meringue is cool and holds a stiff peak.
Add the butter, 1 tablespoons at a time, while beating on medium speed. Scrape down the bowl, add the chocolate, vanilla, espresso and rum (if using), and mix for 1 minute, or until the chocolate is completely blended. If the buttercream seems very soft, allow it to cool and beat it again.
6. To frost the cake, place one cake layer on a serving plate, flat side up. Frost the top of that layer with buttercream. Place the second layer on top, also flat side up and frost the top and sides.