Place the 16-ounce package of powdered sugar and the 6 tablespoons (¾ stick) softened butter in a medium mixing bowl.
Using an electric mixer on low speed, beat the sugar and butter just until they begin to combine and the sugar is incorporated.
Add 1½ teaspoons vanilla extract, 1/8 teaspoon salt, and 3 tablespoons milk. Increase the mixer to medium speed and beat, scraping the sides and bottom of the bowl as needed.
If the icing is too thick for spreading, add up to 1 additional tablespoon of milk, a little at a time, until the icing reaches an easy-spreading consistency.
Beat the icing until smooth and homogenous, scraping the bowl once or twice to ensure even mixing.
Use the icing immediately to spread on cakes or cupcakes. Note: if you pipe rather than spread, you may not have enough to ice 24 cupcakes.