For the cupcakes:

1. Preheat oven to 325 degrees. Line 24 muffin tins with paper liners.
2. In a medium-sized bowl, whisk together flour, baking powder, and salt.
3. In the bowl of an electric mixer, cream together butter and sugar on medium-high speed until light and fluffy (about 3 minutes).
4. Add eggs one at a time, beating thoroughly after each addition. Mix in lime zest and juice.
5. With the mixer on low, add the flour mixture in 3 batches, alternating with the buttermilk — starting and ending with flour. Mix until just incorporated; do not overbeat.
6. Fill each muffin cup about 3/4 full and bake for 25 minutes, or until cake tester comes out clean.
7. In a small bowl, combine tequila and triple sec.
8. When cupcakes are finished baking, brush the liquor mixture onto the top of each cupcake. Allow cupcakes to cool completely before frosting.

For the tequila-lime buttercream:

9. Place the electric mixer bowl over a pot of simmering water. Whisk the sugar and egg whites together until the sugar is dissolved (rub some between your fingers to feel if there are any sugar granules left, or until candy thermometer reaches 160 degrees).
10. Place bowl on the stand mixer fitted with the whisk attachment, and whisk on high speed until the mixture is cool and holds stiff, glossy peaks (about 10 minutes).
11. Change to the paddle attachment, reduce speed to medium, and beat in butter 1 tablespoon at a time. The mixture might appear curdled but keep beating and it will come together.
12. Add lime juice and tequila and increase speed to high until frosting is smooth and fluffy.
13. Pipe onto cupcakes and garnish as desired.