1) In a large mixing bowl, combine the egg whites, sugar, and salt.
2) 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.
3) Attach the bowl to your mixer and beat the meringue with the whisk attachment until it's stiff.
4) 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.
5) 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.
6) Beat in the flavoring of your choice.
7) If the frosting seems soft, chill for 15 minutes before using.
8) Store for up to 6 months in the freezer, or up to 1 week in the refrigerator.
Yield: 5 cups frosting.