Directions

1. In a small saucepan over medium heat, bring the sugar and 1/3 cup water to a boil, and boil until syrup reaches soft-ball stage (a candy thermometer should read 238 degrees).
2. In the bowl of an electric mixer fitted with the whisk attachment, beat the egg whites on low speed until they are foamy. Add the cream of tartar, and beat on medium high until the mixture itself is stiff but not dry.
3. With the mixer running, pour the sugar syrup into the egg-white mixture in a steady stream, and beat on high speed until steam is no longer visible, about 3 minutes. Beat in butter, piece by piece, on medium speed.
4. Add the vanilla extract, and beat for 3 to 5 minutes, until the frosting is smooth and spreadable. If the frosting looks curdled at any point during the beating process, keep beating to smooth it out. If the frosting becomes too soft for piping, stir it over a large bowl of ice water until it stiffens.