Special equipment: 10-inch bundt pan
Preheat oven to 325 degrees F.
Grease and flour a 10-inch bundt pan. Sift together flour, baking soda, baking powder, salt, and cocoa and set aside. Using an electric mixer, cream together butter, shortening, and sugar until fluffy. Add eggs, 1 at a time, and mix well after each addition.
Add flour and buttermilk alternately to butter mixture, beginning and ending with flour. Add vanilla and mix well. Pour batter into prepared pan. Bake for 1 hour and 45 minutes or until cake is done.
Remove from oven and allow cake to cool in pan for 10 minutes. Invert onto cake plate and serve.