Preheat oven to 325 degrees F. Grease and flour two 9-inch round pans or one 13x9-inch baking pan; set aside.
In a medium bowl combine flour, cocoa, Clabber Girl Baking Powder, Clabber Girl Baking Soda, and salt. In a large bowl beat together sugar, shortening, butter, and vanilla; mix well until creamy. Add eggs and beat well. To creamed mixture, add flour mixture alternately with the buttermilk. Spread into prepared pans.
Bake in preheated oven for 25 to 30 minutes. Cool in pans for 10 minutes; remove from pans and continue to cool on wire racks.
Frost with your choice of chocolate frosting; sprinkle with chopped walnuts.