Preheat oven to 300°F.
Use an electric mixer to mix both butter and cream cheese until creamy. Gradually add sugar and beat for 5 minutes, then add eggs one at a time, allowing each to incorporate before adding more. Stir in vanilla.
Sift flour and salt and add gradually to the mixer, beating until well blended. Pour batter into a well greased and floured bundt pan or tube cake pan.
Place 2 cups water in an ovenproof container on the rack in the oven next to the cake. Bake cake at 300 for an hour and 45 minutes, or until a toothpick can be removed cleanly.
Place pan on a wire rack and cool for 15 minutes, then remove the cake from the pan and complete cooling on the wire rack.