In a large mixing bowl, beat together the sugar, butter, lemon zest, and salt until smooth and creamy, about 1-2 minutes.
Scrape bowl down with a rubber spatula. Add the lemon juice and vanilla. Beat for another 1-2 minutes or until light and fluffy.
If the frosting is too thick, add some more lemon juice or milk. If the frosting is too thin, add a little more sugar.
Decorate cake as desired or refrigerate in an airtight container for up to 2 days.