Take the chestnuts and make an incision right through the outer shell of at least one centimetre. Boil them for 20 minutes and leave them to cool in their cooking water. Once they’ve cooled down, remove the outer shell and mash lightly with a fork.
Put the mashed chestnuts back in a pan with milk, the vanilla pod (first cut it in half lengthways, scrape out the vanilla seeds with the back of a sharp knife and add this to the milk along with the outer part of the pod itself) and sugar.
Cook for another 20 minutes, stirring. The chestnuts will absorb all the milk leaving a thick, creamy mixture. Remove the vanilla pod. Mix in cocoa and leave the mixture on a very low flame for another couple of minutes to dry it a bit more. Once the mixture has cooled, leave it in the fridge for one hour.
Take the cold chestnut mixture out of the fridge and pass it through a potato press (so that it resembles spaghetti) onto a serving plate. Build up the chestnut mixture on the plate so that it forms a ‘mountain’.
Whip the cream, sweetening it with icing sugar.
Cover your ‘chestnut mountain’ with whipped cream and decorate with marrons glacè and dark chocolate shavings.