Day 1: Make a syrup by boiling 660mL of water with 500g of sugar and the scraped vanilla pod for 4min, until the sugar is dissolved. Take off the heat, add the chestnuts. Leave to rest for 24 hours.
Days 2-5, each day: Remove chestnuts. Add 90g of sugar, boil to make a syrup (for 4 minutes). Take off the heat. Add the chestnuts. Leave to rest for 24 hours.
On each subsequent day, add sugar, boil and leave to rest. This increases the concentration of sugar in the syrup and allows it to penetrate the chestnuts more deeply.
Day 6: Remove chestnuts. Add 170g of brown sugar and 2 teaspoons of rum, boil to make a syrup (for 4 minutes). Take off the heat. Add the chestnuts. Leave to rest for 24 hours.
Day 7: Remove the chestnuts. Leave to dry for 2 hours. Make a glazing with the syrup: 100g of icing sugar to 50g syrup. Heat the oven to 210°C, put the chestnuts in for 1 min (just enough to warm them). Glaze them while warm, then put them back in the oven for 20-30s.
The glazing is optional: however, if you're going to do it, respect the given steps. The two hours ensure the syrup dries off the chestnuts, and the extremely brief cooking times in the oven are just so the chestnuts are the right temperature for the glazing to set. You don't want to cook them
Recipe by Aliette de Bodard at https://www.aliettedebodard.com/recipes/candied-chestnuts-marrons-glaces/