Cream the butter in the bowl of a stand mixer fitted with a paddle attachment on medium speed.
Gradually add the powdered sugar, and beat on low speed. Mix until the sugar and butter are combined.
Add the pure vanilla extract and the salt to the frosting mixture. Continue to mix on medium-high speed until the frosting becomes smooth.
Add the milk to adjust the thickness of the frosting. If you desire a thinner consistency, add more milk, 1 tablespoon at a time. If the frosting becomes too thin, simply add more powdered sugar.
The frosting can be used to frost a 9-by-13-inch cake, to coat the sides of an 8- or 9-inch layer cake, or to top 12 cupcakes. Make sure the cake is completely cool before attempting to coat it with frosting.
You can make this frosting the day before you use it and store it in an airtight container in the refrigerator. Stir the frosting before applying it to a cake.