Stir together cocoa and boiling water until smooth; set aside. Cream shortening, sugar and vanilla in large bowl until light and fluffy. Add eggs; beat well.
Combine flour, baking soda and salt. Add buttermilk alternately with flour mixture to creamed mixture. Mixing after each addition. Blend in cocoa mixture.
Pour into 2 greased and floured 9-inch round cake pans. Bake at 350 for 30 to 40 minutes. Cool 10 minutes; remove from pans and cool completely. Frost with frosting.