Combine all of the ingredients in a glass or ceramic bowl.
Beat on low using a handheld mixer for approximately 30 seconds (just enough to combine).
Place the bowl over a pot of simmering water and cook, beating on high with your mixer for approximately 7 minutes (the frosting should reach approximately 160F).
Remove from the heat and beat for 1-2 minutes more, or until cooled and holding stiff peaks.
Frost using an offset spatula or spoon into a piping bag.
Notes
How much to make: Not sure how much frosting you'll need? Check out my guide to frosting cakes for estimates.
Start with a perfectly clean bowl for meringues:Fats interfere with the whipping process, so it's important to start with well-cleaned equipment. Warm water and soap work well.
Don’t use plastic bowls to make meringue: It’s very hard to completely remove fats from plastic bowls and kitchen utensils, which tend to develop a greasy coating over time. Instead, use a glass, metal, or ceramic bowl.
Crack your eggs into a separate bowl: Separate the egg yolks from the whites over a small bowl to prevent any fat from hindering the whites' ability to whip to stiff peaks.