Sour Cream Chocolate Cake

1/4 cup unsweetened cocoa powder
1/4 cup boiling water
2 cups sugar
1/2 cup (1 stick) butter, softened
1 cup (8 ounces) sour cream
1 1/2 tsp. vanilla extract
1 tsp. baking soda
2 cups all-purpose flour
1/2 tsp. salt
5 egg whites
2 (16 ounce) cans chocolate frosting

1.) Preheat oven to 350.  Grease and flour two 9-inch round cake pans.
2.) Combine the cocoa powder and boiling water in a bowl and stir until the cocoa is dissolved, about 30 seconds.
3.) Combine the sugar and butter in a large bowl.  Beat with an electric mixer until light and fluffy, 1-2 minutes.  Add the cocoa mixture and mix for 1 minute longer.  Beat in the sour cream, vanilla, and baking soda.  Add the flour and salt.  Mix for 1 minute more.
4.) Beat the egg whites in a medium bowl until stiff peaks form, 2-3 minutes.  Fold into the batter.
5.) Spoon the batter into the prepared cake pans.  Bake until a toothpick inserted into the center comes out clean, 25-28 minutes.  Transfer to wire racks to cool.  Cut each layer into 2 even layers for a total of 4 layers.  Spread the frosting between the layers and over the top and side of the cooled cake.