Preheat oven to 325 F (160 C). Grease and flour a 10 inch (25 cm) tube pan or bundt pan or two 9 x 5- inch (23 x 13 cm ) loaf pans. Sift cocoa in a small bowl. Stir in flour, baking powder and salt; set aside.
In large bowl, beat butter and mayonnaise until smooth. Gradually beat in sugar; beat at high speed until light and fluffy, about 3 minutes. Blend in vanilla. Add eggs, one at a time, beating well after each addition. Stir in flour mixture and milk alternately, beginning and ending with flour. Spread in prepared pan. Bake 55 to 65 minutes for loaf pans, 75 to 85 minutes for tube or bundt pan, or until wooden pick inserted in center comes out clean. Cool in pan on wire rack 10 minutes. Loosen cake from pan and remove. Cool completely. Wrap in foil and let stand at least overnight. For a decorative effect, place a paper doily on top of cake and sprinkle with icing sugar. Remove doily.