Start by placing your room temperature unsalted butter in a large mixing bowl. Using a hand mixer, beat the butter on medium speed for about 1 minute until it's smooth and creamy.
Next, add the lime zest, vanilla extract, and fine salt to the butter. Mix these ingredients on low speed until they are just incorporated, which should take about 30 seconds.
Gradually add in the powdered sugar along with the fresh lime juice while continuing to mix on low speed for about 1-2 minutes until fully combined.
Check the texture of your lime buttercream; if it’s too stiff, add more lime juice a teaspoon at a time until you reach your desired consistency.
For an extra smooth finish, use a spatula to gently stir the buttercream by hand for an additional minute or two.
To keep your lime buttercream fresh, cover the bowl with plastic wrap or transfer it to a piping bag.