Mix 3 cups sugar with water in a small, heavy saucepan. Cook over high heat until syrup reaches 248 degrees F on a candy thermometer. Watch it closely as it gets close to temperature; it will go quickly at the end.
While syrup is cooking, beat egg whites with 2 tablespoons sugar until stiff but not dry.* (Extra- fine granulated sugar works best). Turn mixer off until syrup is ready.
With mixer running, pour syrup into the egg whites in a thin, steady stream, making sure not to pour directly onto beaters. Be careful, as hot syrup can burn if you. Continue to beat mixture until it is no longer hot.
Allow to cool to room temperature and then beat in the softened butter, 1 tablespoon at a time, until all is incorporated. (Salted butter works fine. If you are using unsalted butter, add 1/2 teaspoon salt, or to taste.) The mixture will become thin and may look curdled at first, but will then become smooth as it beats. Lastly, beat in the vanilla all at once. (Use the best vanilla you can find. Artificial and white "vanilla" will ruin the delicate flavor.)
If not using at once, refrigerate the buttercream until needed. It will keep for 1 week in a tightly sealed container in the refrigerator. To use, just bring it back to room temperature and re-beat to restore the texture. If you have trouble with the texture, getting it to room temperature and extra beating are the keys to making it smooth. Enjoy on your favorite cake!
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
Recipe courtesy of Kim Morrison's Cakes for Occasions, Spring Mills, PA