This fluffy, sugary frosting capped off every birthday cake I had growing up. My mother still frosts most of her layer cakes with it: With just egg whites and sugar, it's far less decadent and heavy than a buttercream but has an ethereal quality that makes it much more worthy of a celebration cake in my opinion! —Posie (Harwood) Brien
Combine all the ingredients in the top of a double boiler. Beat with an electric mixer set at low speed for 30 seconds, then set the pan over rapidly boiling water.
Using a handheld mixer, beat the frosting for exactly 7 minutes.
Remove the frosting from the heat, and beat for 1 minute more. If the frosting is too liquid-y and doesn't hold a soft-to-stiff peak, let it cool at room temperature for 10 minutes (or in the fridge for a few minutes), and then beat it on high speed for a few minutes until the texture is thick enough.