In a small clean dry saucepan, place 2 cups sugar and then pour 1/2 cup water down the sides of the pan. Make an X in the sugar with 1 finger to encourage the water to seep into the middle of the pan of sugar. Bring to a boil and cook until softball stage or 235 degrees F on a candy thermometer.
Meanwhile, in a mixer with a whip attachment, whip the whites until they form soft peaks. When the syrup is up to temperature, drizzle it down the wall of the mixing bowl with the mixer running. Continue whipping until light, fluffy and almost cooled down. Start adding the butter a few cubes at a time to cool the frosting and thicken it. When it gets thick you can stop adding butter; you may not use the whole pound (it depends on how much water you cooked out of your syrup). Add vanilla extract to taste.
Recipe courtesy of Gale Gand