Melt the dark chocolate in a bowl suspended over a pan of hot, but not boiling, water. Leave to cool a little.
Melt the white chocolate in a similar way and leave to cool a little.
Whisk the egg yolks and sugar with electric beaters for about 5 minutes until thick, pale and tripled in volume.
Whisk the cream until thick, but be careful not to overdo it.
With clean beaters, whisk the two leftover whites until stiff.
Fold the cream into the egg yolkd mixture, followed by the egg white.
Divide the mixture into two. Fold the white chocolate into one half and the dark chocolate into the other. The dark chocolate will start to reform as it hits the cold mousse, giving you tiny chocolate chips.
Lightly swirl the two mousses together and spoon into four glasses.
Decorate with dark chocolate shavings if desired.
If you have trouble melting the white chocolate, then once you've whisked the yolks and sugar, remove half of the mixture and whisk in the soft chocolate to the remainder. Green & Black's never seems to work for me. It goes soft, but doesn't actually melt.You'll then need to divide the cream and egg whites and add it separately to the white chocolate mixture.