Break the chocolate into small pieces and put in a heatproof bowl. Place in a bain-marie (over a saucepan of simmering water) until almost melted. Heat the cream in a saucepan until just simmering, then add to the melted chocolate and blend until a smooth thick sauce has formed.
Pour into small ramekins or glasses, and leave to chill in the fridge overnight. Garnish with a chocolate scroll or a teaspoon of creme fraiche.