Adapted from traceysculinaryadventures.blogspot.com
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until smooth, about 30 seconds. With the mixer on low, slowly add 2 3/4 cups confectioners' sugar a little at a time until completely incorporated and the mixture is smooth. Scrape down the sides of the bowl then beat in the lime juice, 1 tablespoon of tequila and the salt. Increase the mixer to medium speed and beat until the frosting is light and fluffy, about 4 minutes, stopping once to scrape down the sides of the bowl. Taste the frosting and decide if you want to add the remaining tablespoon of tequila. If so, beat to incorporate. If you want a slightly firmer consistency for piping, add the remaining 1/4 cup confectioners' sugar to the frosting. The frosting can be made a few days ahead and refrigerated. When you are ready to use it, let it come to room temperature and then beat briefly before piping. Frost the cooled cupcakes with the buttercream. Garnish with lime zest or lime wedges, if desired.