Line a loaf tin with clingfilm.
For the white chocolate ice cream
Put 30 g sugar into a pan with the water and bring to the boil.
Beat the egg yolks with the rest of the sugar until foamy, then slowly add the hot sugar water in a thin stream and continue beating until the mixture has cooled.
Roughly chop the white chocolate couverture and melt over a hot bain-marie.
Whip the cream until stiff and fold into the mixture.
Turn the mixture into the loaf tin and freeze for about 3 hours.
For the dark chocolate ice cream
Make the dark chocolate ice cream in exactly the same way, using the dark chocolate couverture and rum.
Turn the mixture onto the frozen white ice cream and smooth the top.
Freeze for about 2 hours then lay the sponge fingers on top and freeze for at least a further 2 hours.
To serve, turn out of the tin, pull off the clingfilm and serve garnished with chocolate curls.