Put the sugar and egg whites in a heatproof bowl set over a pan of simmering water. Whisk until it reaches at least 60˚C/140˚F on a sugar thermometer. Pour into a bowl of an electric mixer fitted with a whisk attachment (or use an electric whisk and bowl). Beat until the mixture has doubled in volume, cooled and will stand in stiff peaks – this will take about 3 minutes. Gradually add the butter to the cooled meringue mix, beating constantly, until the frosting is smooth. Fold in the vanilla and use immediately.
Melt 150g (51/2oz) chopped dark chocolate, then leave to cool slightly before stirring into the buttercream at the same time as the vanilla extract.