Preheat oven to 350 degrees. Grease and flour a 10-inch tube pan. Knock out excess flour. In a large bowl, pour boiling water over chocolate and butter; let stand until melted. Stir in vanilla and sugar, then whisk in egg yolks, one at a time, blending well after each addition. Mix baking soda and sour cream together and add to chocolate mixture. Sift together flour and baking powder; add to batter, mixing thoroughly. In a separate bowl, beat egg whites until stiff but not dry. Stir a quarter of the egg whites into the batter. Scoop remaining egg whites on top of batter and gently fold together.
Pour batter into prepared pan. Set on the middle rack of oven and bake 40-50 minutes, or until the edges have pulled away from the sides of the pan and a cake tester inserted into the center comes out clean. Cool cake in pan for 10 minutes; unmold and cool completely before frosting. 12 servings.
Place all ingredients in a heavy saucepan over low heat and whisk until smooth. Cool slightly; add more sugar if necessary to achieve a spreading consistency. Spread on cake while frosting is still warm.