In a large mixing bowl, combine the egg whites, sugar, and salt.
Place over a saucepan of simmering water (you should just barely see some lazy bubbles coming up), and whisk constantly until the mixture measures 161°F (71.7°C) on an instant-read thermometer.
Attach the bowl to your mixer and beat the meringue with the whisk attachment until it's stiff.
Add the butter a few tablespoons at a time, with the mixer running. Let each blob of butter get completely mixed in before adding the next.
Once half the butter is in the bowl, stop and scrape the sides and bottom to make sure all the meringue is getting incorporated. Then finish adding the rest of the butter.
Beat in the flavoring of your choice.
If the frosting seems soft, chill for 15 minutes before using.
Store for up to 6 months in the freezer, or up to 1 week in the refrigerator.
Tips from our Bakers
The classic flavor for this frosting is vanilla: add 1 teaspoon to 1 tablespoon vanilla extract or vanilla paste, to taste. Or try these flavors:
*For coconut: Add 1/2 teaspoon coconut flavoring and 1/2 cup coconut milk powder.
*For chocolate: Reserve 3/4 cup (6 ounces) of the butter from the recipe. Melt 1 cup (6 ounces) bittersweet chocolate and cool to room temperature. Combine the melted chocolate and the butter until mixed, then add this mixture to the frosting.
*For raspberry: Add 2 tablespoons raspberry purée, or simply mix a half pint of fresh raspberries into the finished frosting.
*For lemon: Add 1 tablespoon fresh grated lemon rind (zest) and 2 teaspoons fresh lemon juice. You can also substitute 1 tablespoon lemon juice powder for the zest and juice, adding it to the egg whites before beating.
Baking vegan? A good substitute for the egg whites in this recipe is aquafaba, the liquid drained from a can of chickpeas. Surprisingly, it whips into peaks just like egg whites! Substitute 2 tablespoons aquafaba for each large egg white; for specifics on the technique read our blog post, A guide to aquafaba.