Melt the chocolate and butter in a heatproof bowl over a pot of simmering water until smooth. Let cool slightly.

Whip the egg whites with a pinch of salt until soft peaks form, then gradually add sugar and whip to stiff peaks.

In another bowl, whisk the egg yolks with vanilla until pale. Slowly mix in the melted chocolate.

Gently fold in the whipped egg whites in thirds to maintain airiness.

Whip the heavy cream until soft peaks form and fold into the chocolate mixture.

Spoon the mousse into serving dishes and refrigerate for at least 2 hours until set. Garnish before serving.