In a saucepan, stir together sugar and water over medium heat. Once dissolved and starts to boil, do not stir anymore. Allow to a boil until a candy thermometer registers 245°F, about 5 minutes.

While the sugar syrup boils, whip the egg whites and cream of tartar in a large mixing bowl until soft peaks form.

With your mixer running at high speed, slowly pour in a thin stream of the hot syrup into the egg whites. After about 5 minutes the frosting should be thick and glossy. Stir in the vanilla, then frost cake.