Put the butter, sugar and cream in a saucepan over a low heat. Leave until it comes to the boil and the sugar has melted. Break the chocolate into small pieces and tip into a heatproof bowl. Pour the butter mixture over the dark chocolate and stir until the chocolate is melted and shiny. Leave to cool, cover with clingfilm and refrigerate until firm. Making one truffle at a time, scoop a teaspoonful of the chocolate mixture and roll between your hands into a ball. Place on a baking tray lined with baking parchment.