In a small saucepan over medium heat, dissolve the sugar in the water. Stirring constantly, bring mixture to a boil and cook until 240˚ is reached on a candy thermometer.
Beat the egg whites until they are foamy. With the mixer running, add the hot sugar syrup to the egg whites in a thin stream. Temper the mixture slowly and carefully.
Add the softened butter, 1 tablespoon (or cube) at a time.