In a large mixing bowl, beat together the butter, cream cheese, vanilla, and salt. Add two cups of the powdered sugar. Beat until smooth. Add more powdered sugar, and beat until fluffy. Continue adding sugar in doses until it has all been incorporated. If your frosting is too stiff, add milk 1 tablespoon at a time until the desired consistency has been reached.

Frost your cake, and store any leftover frosting in an air tight container in the fridge for up to 2 weeks.