Place the chocolate in a heatproof bowl. Place the cream in a small saucepan and bring to the boil, then pour over the chocolate and stir until smooth.
Place the remaining ingredients and 300 ml (10½ fl oz) water in a small saucepan and, using a stick blender, blend until smooth. Bring to the boil, stirring continuously, then remove from the heat and pour over the chocolate mixture. Working quickly, to avoid the mixture becoming too hard, stir well to combine, then pass through a fine sieve into a 30 × 20 cm (12 × 8 inch) tray lined with plastic wrap. Refrigerate for 1 hour or until set.
This can be made 1 day in advance. When ready to serve, cut lengthways into 5 mm (¼ inch) thick slices. You will need 6 slices.