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
To make the cupcakes, preheat the oven to 325˚ F. Line two cupcake pans with paper liners. In a medium bowl, combine the flour, baking powder and salt; stir with a fork to blend. In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium-high speed until light and fluffy, 3-5 minutes. Blend in the eggs one at a time, mixing well after each addition and scraping down the sides of the bowl as needed. Beat in the lime zest, lime juice and vanilla. With the mixer on low speed, mix in the dry ingredients in three additions alternating with the buttermilk, beginning and ending with the dry ingredients. Beat each addition just until incorporated.
Divide the batter between the prepared cupcake liners, filling each about ¾ full. Bake 20-24 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan 5-10 minutes, then transfer to a wire rack to cool completely. If desired, brush the cooled cakes with tequila to emphasize the true margarita flavor.
To make the frosting, combine the sugar, egg whites, and salt in a heatproof bowl set over a pot of simmering water. Heat, whisking frequently, until the mixture reaches 160° F and the sugar has dissolved. Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes. Reduce the speed to medium and add the butter, 2 tablespoons at a time, adding more once each addition has been incorporated. If the frosting looks soupy or curdled, continue to beat on medium-high speed until thick and smooth again, about 3-5 minutes more (don’t worry, it will come together, though it may take a very long time!) Stir in the lime juice and tequila and mix until fully incorporated and smooth.
Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes. Frost the cooled cupcakes with the buttercream. Garnish with lime zest and lime slices as desired.