Preheat the oven to 350 degrees F. Grease a 10-inch tube pan with butter.
Sift the flour, baking powder, salt and cocoa together. Set aside.
With an electric mixer, cream the 1 cup butter, the shortening and the sugar until fluffy. Add the eggs one a time, beating well after each addition. With the motor running, add the flour mixture and the milk alternately, beginning and ending with the flour. Add the vanilla.
Pour the batter into the prepared pan and bake for 1 hour. Check for doneness by inserting a toothpick into the cake; it should come out clean. Cool the cake in the pan for 30 minutes before turning it out onto a wire rack to cool completely.
Slice the cake and serve topped with a scoop of vanilla ice cream.
Adapted from "Georgia Cooking in an Oklahoma Kitchen" by Trisha Yearwood (c) Clarkson Potter 2008. Provided courtesy of Trisha Yearwood. All rights reserved.