Cream butter; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition.
Combine flour, cocoa, soda and salt. Add to creamed mixture alternately with sour cream, beginning and ending with flour mixture. Mix well after each addition. Add boiling water, and mix well. Stir in vanilla.
Pour batter into a greased and floured 10-inch tube pan. Bake at 325 degrees for 1 hour and 20 minutes or until cake tests done with toothpick. Cool cake in pan 10-15 minutes; remove from pan, and cool completely.