Preheat the oven to 300°. Beat butter and cream cheese at medium speed for at least two minutes.
Gradually add sugar, beating 5-7 minutes. Add eggs, one at a time, beating just until yellow disappears. Add vanilla and mix well.
Combine flour and salt in a separate bowl. Gradually add to butter mixture, beating at low speed until blended after each amount.
Pour batter into greased and floured 10 in. bundt pan. Fill a 2 cup ovenproof measuring cup with water, put in the oven with the pan to keep the cake moist.
Bake for 1 ½ hours. Cool in pan on wire rack for 10-15 minutes in the pan, then flip from the pan and allow to cool completely before serving.