1. Heat the water and sugar, stirring constantly, until the sugar has dissolved. Leave to cool.
2. Melt the chocolate in the top of a double boiler. Add the cooled syrup, stirring constantly.
3. Stir in the rum, brandy or Grand Marnier and 45 ml (3 tbsp) cream.
4. Arrange half of the sponge fingers in the bottom of a serving dish. Carefully sprinkle some of the coffee over the sponge fingers, enough to just moisten them. Spread half of the chocolate mixture over the top.
5. Arrange a second layer of sponge fingers gently over the chocolate. Sprinkle with coffee and spread with chocolate as before.
6. Whisk the remaining cream until it is just firm. Spread over the top and sides of the cake and chill for 1 hour.