How to Make It
Heat the oven to 350°F. Line a standard 12-cup muffin tin with paper liners. Combine the flour, the baking powder, and the salt in a medium bowl. Add the butter, sugar, and lime zest to a large bowl and beat with an electric mixer until fluffy, about 4 minutes. Add the eggs and egg yolks, one at a time, then the vanilla. Add the flour mixture and the buttermilk into the sugar and egg mixture in increments, starting and ending with the flour mixture. Stir the dry mixture into the wet mixture.
Divide the batter between the liners. Bake until a toothpick inserted into the center of the cupcakes comes out with just a few moist crumbs, 22 to 25 minutes. Transfer cupcakes to a rack to cool completely.
Prepare the meringue frosting: Whisk the sugar and egg whites in the bowl of a stand mixer or a large bowl. Set the bowl over a pot of barely simmering water, making sure the bottom of the bowl doesn’t touch the water. Continue whisking until the sugar is completely dissolved and the mixture is warm. Remove the bowl from the heat and using the stand mixer or a hand mixer, beat the mixture until it forms stiff, glossy peaks and has cooled to almost room temperature, about 5 minutes. Top the cupcakes with the meringue frosting. If desired, set the cupcakes on a baking sheet and bake in a 500°F oven until the tops and slightly golden and toasted, 2-3 minutes.