Frosted Chocolate Cake

Directions

• 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.
• Preheat oven to 375°. Cream butter, brown sugar and 3/4 cup sugar until light and fluffy. 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.
• With clean beaters, beat egg whites on medium speed until soft peaks form. Gradually add remaining 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.
• Cool in pans 10 minutes before removing to wire racks; remove parchment. Cool completely.
• Beat together first 4 frosting ingredients. Spread between layers and over top and sides of cake. If desired, dust top lightly with cocoa.