Preheat your oven to 180°C (350°F).
Melt the dark chocolate and butter together in a heatproof bowl over simmering water.
In a separate bowl, whisk the eggs and sugar until pale and fluffy.
Stir the melted chocolate mixture into the egg mixture and add vanilla extract.
In another bowl, whip the heavy cream until soft peaks form.
Gently fold the whipped cream into the chocolate mixture.
Pour the mousse into a prepared cake pan.
Chill in the refrigerator for at least 4 hours or until set.
Once set, carefully remove the cake from the pan.
Decorate with chocolate shavings or whipped cream before serving.