Make the Cupcakes: Preheat the oven to 325 degrees F. Line a standard muffin tin with paper liners; set aside.
In a medium bowl, whisk together the flour, baking powder and salt; set aside.
Using an electric mixer on medium-high speed, beat the butter and sugar together until pale, light, and fluffy (about 5 minutes).
Reduce the mixer speed to medium and add the eggs one at at time, mixing thoroughly after each addition.
Scrape the sides of the bowl and add the lime zest, lime juice, vanilla extract and tequila. Mix until combined. (The mixture may start to look curdled at this point, but don't worry, it will all come back together!)
Reduce the mixer speed low. Add the dry ingredients in three batches, alternating with the buttermilk in two batches. Mix only until just incorporated, using a rubber spatula to give it one last mix by hand.
Divide the batter between the muffin cups. Bake until the cupcakes are light golden brown on top and a skewer inserted in the center has moist crumbs attached, about 25 minutes, rotating the pan halfway through baking.
Allow cupcakes to cool for 5 to 10 minutes, and then remove to a cooling rack. Brush the tops of the cupcakes with the 1 to 2 tablespoons of tequila. Set the cupcakes aside to cool completely before frosting them.
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. Increase speed to medium and mix for 30 seconds. Add the lime juice, tequila and salt, and beat 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.