Put the chocolate, sugar, butter, vanilla and three tablespoons of water into saucepan. Heat gently until the sugar has dissolved, then add canned chestnuts purée. Remove the pan from the heat, mix together until well combined, then transfer to a large bowl and set aside to cool. Whip the double cream in a bowl until soft peaks form when the whisk is removed, then fold into the cooled chocolate chestnuts mixture. Spoon the mixture into eight ramekins, cover with cling film and chill in the fridge for 2-3 hours, or until the mixture has set. To serve, top with a little whipped cream and grate some chocolate over the top. Sprinkle over the orange zest and serve.