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.
Variations: Peanut Butter: Omit the vanilla and increase the milk to 2/3 to 3/4 cup. Substitute 3/4 cup peanut butter for an equal amount of the butter. Milk Chocolate: Fold 4 ounces melted semisweet chocolate into the buttercream. Coffee: Omit the vanilla. Dissolve 1 tablespoon instant espresso powder into the 4 tablespoons milk before adding to the buttercream. Lemon: Omit the vanilla and reduce the milk to 2 tablespoons. Beat 2 tablespoons lemon juice and 1 tablespoon lemon zest into the buttercream. Strawberry: Omit the vanilla, milk and 1 cup of the powdered sugar. Beat 1/2 cup strawberry jam into the buttercream. Champagne: Omit the vanilla and milk. Beat 3 to 4 tablespoons Champagne into the buttercream.