Beat 1 cup of unsalted butter on a medium speed for 1 minute with a hand mixer until smooth.
Add in the zest of 1 small lime, 1 tsp vanilla extract, and 1/4 tsp salt. Mix on a low speed until the ingredients are incorporated.
Mix in 3 1/2 cups of powdered sugar and 1 Tbsp of fresh lime juice on a low speed. Scrape the sides and bottom of the bowl as needed.
Once fully mixed, if the frosting seems too thick, add additional lime juice (1 tsp at a time). If the frosting is too thin, add more powdered sugar (quarter of a cup at a time).
If your frosting looks broken, add in more room-temperature butter, 1 Tbsp at a time until it comes back together.
Stir the frosting by hand with a rubber spatula once it's made for a minute or two to make the frosting super smooth.
Either cover the frosting flush with plastic wrap to prevent crusting or place it in a piping bag fitted with your favorite frosting tip. This buttercream can sit at room temperature for a day, be stored in the fridge for up to a month in an airtight container, or be stored in the freezer for 3 months.