Beat butter and cream cheese until creamy, about 2 minutes..Gradually add sugar and beat 5 minutes. Add eggs one at a time, beating well after each addition. Add vanilla and mix well. Combine flour and salt in a four cup measuring cup and gradually add to butter mixture. Beat at low speed just until blended. Pour batter into the prepared tube pan. Fill a 2 cup measuring cup with water. Place in oven with the cake. Make sure measuring cup is oven proof. Bake at 300 hundred oven for an hour and a half. After removing from pan, place on rack to cool completely.