• Preheat oven to 325 degrees and line a full-sized or mini cupcake tin with cupcake papers.
• In a medium bowl, combine flour, baking powder, and salt together.
• In the bowl of a stand mixer with paddle attachment or in a large bowl with a hand mixer, cream together butter and sugar on medium-high speed until light and fluffy, about 3 minutes.
• Blend eggs in one at a time, mixing well after each addition. Beat in lime juice, lime zest, and vanilla.
• With mixer on low speed, mix in dry ingredients in 3 additions, alternating with buttermilk. Beat each addition until just incorporated, taking care not to over-mix.
• Divide batter into cupcake papers, filling about ¾ full. For full-sized cupcakes, bake 20-24 minutes and for mini cupcakes, bake 11-13 minutes, until a toothpick inserted into the center comes out clean.
• Let cupcakes cool in tin for a few minutes before moving to wire racks to cool completely.
• Once cupcakes are completely cooled, lightly brush them with tequila. Spoon frosting onto cupcakes and top with lime zest, lime edges, and straws, if desired.

Margarita Frosting

• Combine sugar, egg whites, and salt in a heatproof bowl over a pot of simmering water (can use a double boiler). Whisk frequently until sugar has dissolved and mixture reaches about 160 degree F on a thermometer. 
• Put mixture in bowl of stand mixture with whisk attachment. Beat on medium-high speed until stiff peaks form and mixture is cooled. This should take about 8 minutes.
• Reduce mixer speed to medium and add butter, 2 Tbsp at a time, waiting to add another until the previous piece has been incorporated. If mixture seems to be getting soupy or curdled, beat on medium-high again until it thickens up. Stir in lime juice and tequila.