Cream the butter in a stand mixer fitted with the paddle attachment (or in a large bowl if using a handheld electric mixer). On low speed, beat in the powdered sugar and then the salt. Add the vanilla, increase the speed to medium-high and beat until light and fluffy. Beat in 2 tablespoons milk; adjust the consistency with extra milk as desired. Use immediately, or refrigerate in an airtight container for up to 3 days. Allow to come to room temperature and beat on medium-high until smooth.