Melt the dark chocolate and leave to one side to cool. Whisk the egg whites to soft peak stage and set aside.
Place the egg yolks and castor sugar in a large mixing bowl. Whisk on high speed for a few minutes until pale and thick. Whisk in the vanilla extract and cooled chocolate.
Add the cream and whisk until everything is combined. Carefully fold in the egg whites by hand. Lastly, fold in the amaretti biscuits.
Tip the mixture into the prepared tin and smooth the top. Cover and freeze until firm (about 4 hours).
Remove the tin from the freezer just before serving. Carefully turn the semifreddo out onto a serving platter. Top with crushed amaretti biscuits and a dusting of cocoa powder. Slice with a sharp knife and serve immediately.