Using the paddle attachment of your electric mixer, and medium speed, cream the softened butter & cream cheese for about five minutes. Add the powdered sugar and lemon zest, and then continue to beat for 1 minute. Add the vanilla, milk, and a pinch of salt. Beat once more on medium speed for another minute. If the mix seems too thick, add a splash more milk and combine a final time. Place atop the cinnamon rolls while they are still warm.