9. To make the frosting, whip the butter on medium-high speed of an electric mixer using the whisk attachment for 5 minutes. Reduce the speed to medium-low, and gradually add the powdered sugar, mixing and scraping the sides of the bowl until all is incorporated. Give it a mix on medium speed for about 30 seconds. Add the lime juice, tequila and salt mix on medium-high speed until incorporated and fluffy. If the frosting appears a bit too soft, add some additional sugar, one spoonful at a time until desired consistency is reached. Frost cupcakes and garnish, if desired, with lime zest, an additional sprinkling of salt and a lime wedge.