Do NOT preheat oven. Cream the butter and shortening together.
Add the sugar and beat until light and fluffy.
Beat in the eggs, one at a time, beating for about a minute after each egg.
Sift together flour, salt and baking powder.
Add the dry ingredients, a third at a time, to the creamed mixture, alternating with the milk.
Add the vanilla extract.
Pour into a greased and floured 10 inch tube pan.
Place in a cold oven. Set the temperature at 325 degrees and bake for 1 hour and 20 minutes or until a toothpick inserted in the center comes out clean.
For the glaze, place butter and water in a saucepan and heat until the butter melts. Add the sugar and stir while cooking for about 2 minutes. Remove from the heat, add the rum, and spoon over the hot cake while it is still in the pan.