Chocolate Buttercream Cake

Cake:
1 3/4 c. all-purpose flour
1 c. cocoa powder
1 1/2 tsp. baking soda
1/4 tsp. salt
3/4 c. unsalted butter, at room temperature
2/3 c. granulated sugar
2/3 c. light brown sugar, packed
2 extra-large eggs, at room temperature
2 tsp. vanilla extract
1 c. buttermilk, at room temperature
1/2 c. sour cream, at room temperature
1 tsp. espresso powder
2 tbsp. milk

Frosting:
10 oz. bittersweet chocolate
8 oz. semisweet chocolate
1/2 c. egg whites (3 extra-large eggs), at room temperature
1 c. granulated sugar
Pinch of cream of tartar
1/2 tsp. salt
1 lb. unsalted butter, at room temperature
2 tsp. vanilla extract
2 tsp. espresso powder, dissolved in 1 tsp. water

Preheat oven to 350°F. Butter two 8-inch round cake pans. Line bottoms with parchment paper. Butter the paper and dust pans with flour, knocking out any excess.

In a medium bowl, sift together flour, cocoa, baking soda, and salt. In the bowl of an electric mixer fitted with paddle attachment, cream butter and sugars on high speed until light, approximately 5 minutes. Add eggs and vanilla and mix well. Combine buttermilk, sour cream, and coffee. On low speed, add flour mixture and buttermilk mixture alternately in thirds, beginning with buttermilk mixture and ending with flour mixture. Mix the batter only until blended. Divide batter between two pans and smooth tops with a spatula. Bake on middle rack of oven for 25 to 30 minutes or until a toothpick comes out clean. Cool for 10 minutes on a rack, remove from pans, and allow finish cooling.

To make frosting, chop chocolates and melt in a bowl over simmering water until smooth. Allow to cool. Mix egg whites, sugar, cream of tartar, and salt in the bowl of an electric mixer fitted with a whisk. Heat egg white mixture in bowl over simmering water until warm to the touch, about 5 minutes. Whisk on high speed for 5 minutes or until meringue is cool and holds a stiff pea. Add butter, 1 tbsp. at a time, while beating on medium speed. Scrape down bowl; add chocolate, vanilla, and espresso and mix for 1 minute or until chocolate is completely blended in. If buttercream seems very soft, allow to cool and beat again.

To frost cake, place one layer on a serving plate, flat side up. Frost top layer with buttercream. Place second layer on top, also flat side up, and frost top and sides.