* minus 2 Tbls ** Preheat oven to 350~F. Grease and flour a 10-inch tube pan. Knock out excess. 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 and whisk into chocolate mixture. Sift flour and baking pwoder together and add to batter, mixing thoroughly. Beat egg whites until stiff but not dry. Stir a quarter of the egg whites thoroughly into the batter. Scoop remaining egg whites on top of the batter and gently fold together. Pour batter into the prepared pan. Set on the middle rack of the oven and bake for 40 to 50 mins., 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 in pan for 10 mins; unmold and cool completely before frosting. 12 portions. Chocolate Frosting: 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.