Preheat oven to 350F. Grease 2, 9-inch round cake pans; dust pans with flour. Cream butter and sugar together in large bowl. Add eggs; beat until light and fluffy. Beat in vanilla and chocolate. Sift flour, baking soda, and salt together into separate bowl. Alternately add flour mixture and sour cream to butter mixture; beat well after each addition. Stir in boiling water; batter will be thin. Pour batter into prepared pans; bake for 35 minutes. Cool in pans on wire rack for 10 minutes, then turn out onto rack; cool completely before frosting as desired.