This traditional frosting is fluffy, white, and almost meringue-like in texture. It's easy to make and takes 7 minutes to cook.
In the metal bowl of a standing mixer set over (but not touching) a saucepan of simmering water, combine the sugar, corn syrup, water, egg whites, and salt. Cook over medium heat, stirring frequently, until the sugar has completely dissolved, 3 to 4 minutes. Attach the bowl to the mixer fitted with the whisk attachment. Beat the mixture on high speed until glossy, thick, and billowing peaks form, about, well, you know, 7 minutes. (Although truth be told, sometimes it doesn't take quite that long. When in doubt, just look at the picture above; that’s what you want.) Beat in the vanilla. Use to frost a cake or cupcakes immediately. Best when consumed within several hours.