Combine butter, sugar, chocolate, and boiling water in a large mixing bowl; beat well. Add eggs, one at a time, beating well after each addition.
Dissolve soda in buttermilk; set aside. Combine flour and baking powder; add to chocolate mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Mix well after each addition. Stir in the vanilla
Pour batter into 2 greased and floured 9-inch round cake pans. Bake at 325° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove the layers from pans, and let cool completely.
Spread the filling between layers; spread top and sides of the cooled cake with Sour Cream Chocolate Frosting.