Combine the butter, sugar and salt in the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl if using a handheld electric mixer). Mix on low speed until mostly incorporated. Add the vanilla, increase the speed to medium-high and mix until smooth. Adjust the consistency with milk as desired. Use immediately, or refrigerate in an airtight container up to 3 days. Allow to come to room temperature and mix on medium-high until smooth.