Grease and flour a 10-inch bundt pan. Sift flour, baking powder, salt, and cocoa. Set aside.
In a large bowl, cream butter, shortening, sugar, and vanilla until light and fluffy. Add eggs one at a time, beating well after each. Add flour mixture alternately with the milk. Mix well.
Pour into bundt pan. Bake for 70 minutes or until a toothpick inserted into cake comes out clean. Let cool for ten minutes in the pan, then turn out onto a wire rack and cool completely.
Makes 1 10-inch bundt cake, or 12 servings.