1. Sift flour with cocoa, baking powder and salt; set aside.
2. Lightly grease & flour a 10-inch tube pan. Preheat oven to 350° F.
3. In large bowl of electric mixer, at medium speed, beat butter, shortening, sugar until light and fluffy--about 5 minutes.
4. Add eggs, one at a time, beating well after each addition.
5. With mixer at low speed, beat in flour mixture, in 4 additions, alternately with milk, in 3 additions, beginning and ending with flour mixture. Add grated chocolate and vanilla.
6. Turn batter into prepared pan. Bake 1 hr and 15 to 20 min, or until cake tester inserted in center comes out clean.
7. Let cool in pan on wire rack 10 min. Turn out of pan; let cool completely on rack. If desired serve with whipped cream and shaved chocolate. Makes one 10-inch cake.
from McCall's Mag. Apr. 1972