Put the chestnuts and vanilla pod into a heavy-based pan and add enough water to just cover them. Bring to the boil, then reduce the heat, cover the pan and simmer for 30 minutes. Remove the vanilla pod and set aside, then strain and reserve the cooking liquid. Purée the chestnuts in a food processor or blender, adding a little of the reserved liquid if necessary.
Put the chestnut purée back into the pan. Slice the vanilla pod lengthways and scrape the seeds into the pan.
Add the sugar and 75 ml (3 fl oz) of the cooking liquid and stir to blend. Bring to the boil, stirring frequently, and cook for about 5 minutes, or until very thick. Remove from the heat and add the whisky.
Ladle into warm, dry jars. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to mature for 2 days in a cool, dark place.
To serve, layer spoonfuls of jam in small glasses with honey-flavoured natural yogurt.