In the bowl of an electric mixer fitted with the paddle attachment, beat butter for 2 minutes on medium speed.

Adjust speed to low and add powdered sugar to butter, one cup at a time, until well incorporated. Add salt, vanilla extract, and milk or heavy cream. Beat on medium for 3 minutes.

Add additional powdered sugar or milk/cream until desired consistency is reached.

For firmer frosting, add more powdered sugar (1/4 cup at a time). For looser frosting, add more milk or cream (1 tablespoon at a time).