Soak gelatine in cold water. Mix egg yolks, starch and sugar in a metal bowl. Heat the milk to just before boiling point and add it while stirring continuously. Heat slowly over a hot water bath while stirring constantly until the cream thickens (do not boil!).
Squeeze the gelatine, add to the cream and dissolve in it while stirring. Place the bowl in a cold water bath and let it cool down while stirring occasionally.
Whip the double cream and cream together and fold into the cooled cream. Coarsely chop the dark chocolate and melt slowly over a warm water bath, allow to cool briefly. Fold the melted chocolate into 2/3 of the cream.
Pour dark chocolate mousse into 4 glasses (approx. 200 ml each). Coarsely chop white chocolate and melt slowly over a warm water bath, let cool down briefly. Fold chocolate into the last third of the cream.
Pour light mousse over the dark mousse. Chill for about 2 hours.