Cream butter; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition.
Dissolve soda in buttermilk; add to creamed mixture alternately with flour, beginning and ending with flour. Mix well after each addition. Add chocolate; mix well. Stir in vanilla.
Pour batter into a greased and floured 10-inch tube pan. Bake at 350° for 1 hour or until cake tests done. Cool in pan 10 to 15 minutes; remove from pan, and let cool completely.
Spread Special Chocolate Frosting on top and sides of cake.