1. Heat the egg whites and sugar in a pan until the sugar has dissolved and the mixture is hot and steaming (approx. 80°C)
2. Pour into an electric mixer with a whisk attachment fitted and whip for 5minutes until voluminous, white, and holding its shape.
3. Slowly add the butter, incorporating fully between each batch. Stir in the vanilla extract, set aside.
4. Spoon into a piping bag fitted with a round nozzle.