For the cupcakes:
3 cups all-purpose flour
1 tbsp. baking powder
½ tsp. salt
1 cup (2 sticks) unsalted butter, at room temperature
2 cups sugar
4 large eggs, at room temperature
3 limes, zested and juiced
½ tsp. vanilla extract
1 cup buttermilk

For the frosting:
2 cups sugar
8 large egg whites
Pinch fleur de sel (coarse salt)
1½ cups (3 sticks) unsalted butter, at room temperature
2½ tbsp. freshly squeezed lime juice
2-4 tbsp. tequila