1. Preheat the oven to 200C/400F/Gas Mark 6.
2. Roll the puff pastry to a 10cm square, using icing sugar instead of flour to stop it sticking. Once rolled, sprinkle generously with more icing sugar and roll the square up as tightly as possible like a Swiss roll. Slice this into 8 on an angle and roll each piece out as thinly as possible into a long tongue shape.
3. Bake these on a non-stick tray for 15 minutes until dark golden. Halve and core the pears and place them in a baking tray with the butter and chestnuts and bake for 20 minutes.
4. While these are cooking, bring the water for the chocolate sauce to the boil with the sugar and cocoa powder and allow to simmer, stirring continuously for 2 minutes.
5. Remove from the heat, add the chocolate and stir until the chocolate has completely melted.
6. Soften the chestnut puree and mix with the double cream.
7. Serve the pears and chestnuts either hot or at room temperature on the chocolate sauce with the chestnut cream spooned over and the puff pastry on top