Frosted Chocolate Cake

Directions

1. Place egg whites in a large bowl; let stand at room temperature 30 minutes. In a small bowl, mix cocoa and boiling water until smooth; cool slightly. Line bottoms of 2 greased 9-in. round baking pans with parchment; grease parchment.
2. Preheat oven to 375°. Cream butter, brown sugar and 3/4 cup sugar until light and fluffy, 5-7 minutes. Beat in vanilla and 1 egg yolk at a time. In another bowl, whisk together flour, baking soda, cream of tartar and salt. Add to creamed mixture alternately with buttermilk, beating after each addition. Stir in cocoa mixture.
3. With clean beaters, beat egg whites on medium speed until soft peaks form. Gradually add remaining 1/4 cup sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Continue beating until stiff peaks form. Fold into batter. Transfer to prepared pans. Bake until a toothpick inserted in center comes out clean, 23-28 minutes.
4. Cool in pans 10 minutes before removing to wire racks; remove parchment. Cool completely.
5. Beat together first 4 frosting ingredients. Spread between layers and over top and sides of cake. If desired, dust top lightly with cocoa.