Preheat oven to 160C/Gas 3.
1) Grease and flour a 25-cm bundt tin. Sift together flour, bicarbonate of soda, baking powder, salt, and cocoa and set aside.
2) Using an electric mixer, cream together butter, vegetable fat, and sugar until fluffy. Add eggs, 1 at a time, and mix well after each addition.
3) Add flour and buttermilk alternately to butter mixture, beginning and ending with flour. Add vanilla and mix well. Pour batter into prepared tin. Bake for 1 hour and 45 minutes or until cake is done.
4) Remove from oven and allow cake to cool in tin for 10 minutes. Invert onto cake plate and serve.