With a mixer, cream butter and shortening and sugar together.
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.
Grease and flour 2 loaf pans.
Pour ⅓ of batter into loaf pans. Sprinkle half of cinnamon sugar mixure into pans. Repeat with remaining batter, then cinnamon sugar, then ending with batter.
Bake about 1 hour or until toothpick inserted into the center comes out clean.
Topping
In a small bowl whisk together the icing ingredients. Until creamy. Spread over the top of the cooled cake. Allow icing to set up atleast 1 hour before slicing.