Use a little water to dampen a 450g (1lb) loaf tin (the more square shaped, the better), then line with a double layer of cling film. Line 2 large baking sheets with non-stick baking paper.
Melt the plain chocolate in a heatproof bowl set over a pan of simmering water. Spoon half the melted chocolate onto each sheet of baking paper and spread it out to the edges in a thin layer. Put in the fridge for about 30 minutes to set.
Melt the white chocolate and 5 tablespoons of the cream in a separate heatproof bowl set over a pan of simmering water. Once melted, give it a good stir and set aside to cool.
Whip the rest of the cream in a large bowl with the vanilla seeds until the cream is just holding its shape, then fold in the cooled melted white chocolate.
In a separate bowl, whisk the egg white and icing sugar until stiff peaks have formed, then fold into the white chocolate mixture.
Carefully peel the plain chocolate from the baking paper and break it up into pieces.
Place a couple spoonfuls of the whipped cream mixture into the prepared loaf tin and cover with a layer of the plain chocolate pieces. Repeat the layers until the loaf tin is filled up, finishing with a layer of whipped cream. Cover the top with cling film and freeze overnight (or up to 1 month). Any remaining pieces of plain chocolate can be put into a freezer-proof container and frozen separately to use for decoration.
About 1 hour before serving, transfer the loaf tin and reserved chocolate decoration to the fridge.