Melt chocolate in bowl in microwave. Set aside. Grease and flour 2 9" cake pans. Preheat oven to 350. In large mixing bowl, cream butter until smooth. Add sugar and eggs and beat on high speed about 5 minutes til light and fluffy. Use spatula to push down sides of dough occasionally (turn off beater to do this). With mixer on low speed put in vanilla and melted chocolate, baking soda and salt. Add flour alternately with sour cream, beating at low til smooth. Pour in boiling water and stir just til blended. Don't overbeat. Pour into pans and bake 25-30 min or til a toothpick comes out clean. Cool in pans 10 min. Turn out on wire racks. Cool completely. Frost. Makes one 2 layer cake.