Preheat the oven to 180°C.
Butter a 20cm spring-bottomed cake tin.
Melt the dark chocolate and butter in a bowl over simmering water.
Whisk the egg whites until stiff peaks form.
Whisk the egg yolks and caster sugar in a separate bowl until pale.
Add the melted chocolate mixture to the egg yolks.
Gently fold the egg whites into the chocolate mixture.
Pour the mixture into the tin and bake for 20 minutes.
Cool the cake and dust with icing sugar and cocoa powder.
Beat mascarpone, cream, icing sugar, and passion fruit pulp until firm.
Serve the chocolate mousse with the passion fruit mixture and mint.