In a small saucepan over medium heat, bring sugar and ½ cup water to a boil.
Heat up to 230 degrees on a candy thermometer.
Whip egg whites until stiff. Do not over beat.
When the sugar temperature reaches 240 degrees, remove from heat immediately and with the mixer running, add it to your egg whites in a stream. Beat on high speed.
Continue to whip until no heat remains in the bowl.
With the mixer still running, add butter bit by bit, beating until spreadable consistency, approximately 3 to 5 minutes. Beat in vanilla.
Set aside until ready to use.