Preheat oven to 350 degrees
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. It will be a thin batter.
Pour batter into a greased and floured 10 inch tube pan. Bake at 350 degrees for 1 hour and 20 minutes, or until a wooden pick inserted in center comes out clean. Cool cake in pan for 10 to 15 minutes; remove from pan and cool completely. Yield: one 10 inch cake.