With a mixer, cream butter and shortening together. Add sugar, a little at a time. Add eggs, 1 at a time, beating after each addition. Stir dry ingredients together in a bowl and add to mixer alternately with milk, starting with the flour and ending with the flour. Mix in vanilla. Pour into a greased and floured tube pan and bake for 1 to 1 1/2 hours, until a toothpick inserted in the center of the cake comes out clean.