CAKE: Combine, melt and cool chocolate, butter and water. Sift into mixing bowl the flour, baking soda and salt. Mix in separate large bowl the eggs, sour cream, sugar and vanilla. Stir chocolate mixture into egg mixture. Add this to flour mixture one half at a time, just until smooth. (Batter will be thin.) Two 8 inch layers. Bake 35-40 minutes at 350 degrees.
FROSTING: Beat until blended. Cook in double boiler, beating constantly with electric beater 5-7 minutes. Remove from water; stir in vanilla. Melt 1 square chocolate with 1 tablespoon butter. Drizzle over top of frosting letting mixture run down sides.