This frosting is a paradox of sweetness -- it's perfectly airy and yet it maintains a level of structure other buttercreams lack. Pipe or mound this delicious icing over our Lemon Meringue Cake.
Place egg whites, sugar, and vanilla in a heatproof bowl of an electric mixer. Combine, using a wire whisk; continue whisking with the bowl set over a pot containing about 2 inches of simmering water.
Whisk constantly until mixture feels hot to the touch. Transfer to mixer and whip, using the whisk attachment, on high speed until whites are glossy and stand in stiff peaks when the whisk is lifted.
Warming the egg whites helps dissolve the sugar and gives the meringue greater volume.