280g caster sugar
5 large eggwhites
pinch of cream of tartar
454g unsalted butter, chilled
1 teaspoon pure vanilla extract
In a small saucepan over medium heat, bring sugar and water to a boil. Contine boiling until syrup reaches 238 degress (115C) on a candy thermometer (soft ball stage)
Meanwhile, place egg whites in a bowl of a standing mixer fitted with the whisk attachment, and beat on low speed until foamy. Add cream of tartar and beat on medium high speed until stiff but not too dry, do not overbeat.
With mixer running, add syrup to whites in a stream, beating on high speed until no longer steaming, about 3 minutes. Add butter bit by bit, beating until spreadable, 3 to 5 minutes. Beat in vanilla. If icing curdles, keep beating until smooth.