1. In a medium-size bowl, beat together powdered sugar and butter on medium speed. Add milk, vanilla and salt and beat until smooth. Correct consistency, if necessary, adding additional powdered sugar of lliquid of choice.
2. To store, cover surface with plastic wrap. Keep up to 3 days at room temperature, up to 3 weeks refrigerated, up to 6 months frozen. To use, soften and beat until smooth. Makes about 1 cup.