In a large mixer bowl, cream butter until smooth. Add brown sugar and eggs. Beat with mixer till light and fluffy, about 5 min. With mixer on low speed, beat in vanilla and chocolate, then baking soda and salt. Add flour alternately with sour cream, beating on low speed until smooth. Pour in boiling water; stir with spoon until blended. Pour into greased and floured pan(s). Bake 35 minutes or until done. Makes one 9x13" pan or one 2-layer 9" round cake.