In a bowl, cream butter, shortening and sugar. Add eggs, one at a time, beating well after each addition. Add milk and vanilla. Sift together dry ingredients; add gradually to creamed mixture, blending just until well mixed. Pour into a well-greased fluted tube pan. Bake at 350° for about 1 hour (toothpick will not come out clean when testing). Let stand 2-3 minutes before inverting onto a cake plate. Serve warm or cold with whipped cream if desired. Refrigerate leftovers.