In a large bowl, beat the softened unsalted butter on medium speed until creamy, about 2 minutes.
Gradually add powdered sugar, 1 cup at a time, beating on low until combined. Increase speed to medium and beat until smooth.
Add fresh lemon juice, lemon zest, vanilla extract, and salt to the mixture and beat on medium speed until fully incorporated and fluffy.
If frosting is too thin, add more powdered sugar 1 tablespoon at a time until desired consistency is reached.
Use immediately to frost cakes or cupcakes, or store in an airtight container in the refrigerator for up to 3 days. Bring to room temperature and remix before using.
Notes
For a more intense lemon flavor, add extra lemon zest or a few drops of lemon oil. Make sure butter is softened to avoid lumps.