Beat butter and cream cheese at medium speed with an electric mixer until creamy (do not over mix); gradually add sugar, beating well.
Add eggs, 1 at a time, beating until combined. (You can crack the eggs into a bowl first before adding to the mixture to make sure you avoid shells in the cake mix.)
Sift 3 cups of flour.
Combine flour and salt; gradually add to butter mixture (I added a cup at a time), beating at low speed just until blended after each addition.
Stir in vanilla. Mix.
Pour batter into a greased and floured 10-inch Bundt pan.
Bake at 300° for 1 hour and 40 minutes or until a wooden toothpick inserted in center comes out clean. (YES, it's 300 degrees for 1 hour and 40 minutes. That is not a typo!)
Cool in pan on a wire rack 10 to 15 minutes; remove from pan, and let cool completely on wire rack.