Cream butter and margarine until smooth; add sugar and beat until light and fluffy. Add eggs, one at a time, beating well after each addition. Combine dry ingredients, and add to creamed mixture alternately with milk and vanilla. spoon batter into a well-greased 10 inch tube pan or Bundt pan. Bake at 325 degrees for 35 minutes; reduce heat to 200 degrees and bake an additional 35 to 40 minutes. Cool; spread with Butter Cream Frosting, and decorate as desired. Butter Cream Frosting: Cream margarine and cream cheese until light and fluffy. Add salt and sugar, a small amount at a time, beating until smooth. Slowly beat in just enough milk to make mixture spreadable; add vanilla.