
Margarita Cupcakes
from Annie's  Eats
Yield: about 24 cupcakes
Ingredients:
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, cold
2½  tbsp. freshly squeezed lime juice
4 tbsp. tequila
Directions: 
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.
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 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.