Stir together cocoa and boiling water until smooth; set aside.
Cream butter, shortening and sugar in large mixer bowl. Blend in salt, eggs and vanilla. Stir baking soda into buttermilk; add alternately with flour to creamed mixture. Blend in reserved cocoa mixture.
Pour batter into two greased and wax paper lined 9-inch round layer pans (two 8-inch square pans or a 13x9x2 inch pan). Back at 350° for 30-35 minutes (40-45 minutes for 8-inch or 13x9x2 inch pans) until cake tester inserted in center comes out clean. Cool 10 minutes; remove from pans. Cool completely, frost as desired.