In a large mixing bowl, cream butter and shortening.
Gradually add 2 cups sugar; cream until fluffy.
Add eggs, one at a time, beating well after each addition.
Combine milk and vanilla; set aside.
Sift together flour, baking powder and salt; add to creamed mixture alternately with milk mixture, beating just enough after each addition to keep batter smooth.
Combine cinnamon and remaining sugar; sprinkle 1-1/2 teaspoons into a greased 10-in. tube pan.
Pour 1/3 of batter into pan.
Sprinkle half of remaining cinnamon/sugar; top with 1/3 of batter.
Repeat with remaining cinnamon/ sugar and batter. Smooth top with spatula.
Bake at 350 degrees F for 1 hour and 15 minutes.
Cool 10 minutes; remove from pan to a wire rack to cool thoroughly.