1/4cuppowdered sugar,only if needed to reach the desired consistency and sweetness
Garnish with lime slices and zest if desired
To prepare the cupcakes:
Line a cupcake pan(s) with paper liners. You will need 12 or 13 lines. Preheat oven to 350 degrees.
In a large measuring cup with a pouring spout, whisk together the egg whites, milk, and vanilla. Set aside.
Using a stand mixer fitted with the paddle attachment, blend the flour, granulated sugar, baking powder and salt on low for about 30 seconds. With the mixer running, add the butter 1 piece at a time until incorporated and the mixture resembles moist crumbs.
Add all but about 1/2 cup of the egg mixture and blend just until combined. Increase the speed to medium-high and beat until light and fluffy, about 1 minute. Add the remaining 1/2 cup of egg mixture in a slow stream. Scrape down the sides and beat on medium-high until well blended. Add the lime zest and juice and mix just until incorporated.
Scrape the batter into a large Ziplock bag and snip off one corner with scissors. Pipe the batter into the liners, dividing evenly between 12 or 13 cupcakes. I made 12 but they were a little big and over the edge, 13 would be about perfect. Bake in a 350 degree oven for 18-20 minutes. Test the cupcakes with a toothpick inserted in the center. If it comes out clean, they are done.
Cool the cupcakes in the pan for 10 minutes. Transfer cupcakes to a rack and let cool completely before icing.
To prepare the Swiss buttercream:
Combine the sugar, egg whites and salt in a heatproof stainless bowl or the pan of a double boiler. Set the bowl over a pot of simmering, not boiling, water. Whisk the egg whites until the mixture reaches 160 degrees on an instant read thermometer and the sugar has dissolved. Don’t get in a hurry here – you don’t want cooked egg whites.
Transfer the egg whites to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high (7 on a KitchenAid Mixer) until stiff peaks form and the mixture has cooled to room temperature, about 8-10 minutes. Reduce the speed to medium (5 on a KitchenAid Mixer) and add the butter 2 tablespoons at a time. Add more after each addition has become smooth and fully incorporated. The mixture may start to look soupy and curdled but continue to beat on medium-high speed until it is thick and smooth again, about 3-5 minutes.
Add 3 tablespoons of lime juice and mix on low until incorporated. Taste the frosting to ensure it is sweet enough and the lime flavor is dominate. If the mixture is not the right consistency, add 1 more tablespoon of lime juice. Mine became thick and almost butter like so I added confectioners’ sugar, 1 tablespoon at a time, until it was smooth and creamy and had the right consistency.
Frost cupcakes using a piping bag and the desired tip to decorate.
Garnish with lime zest and lime slices if desired. The buttercream may be kept at room temperature or transfer to an airtight container and refrigerate up to 3 days. The frosting may also be frozen for up to 1 month. Before using bring the frosting to room temperature and beat on low speed with a mixer to achieve the proper consistency.
(cake slightly adapted from Cook’s Country and the buttercream was adapted from Annie’s Eats)