Make a cross cut in the top of each chestnut then add to a saucepan of boiling water and poach for 15 minutes. Drain into a colander, rinse with cold water so that they are cool enough to handle, then remove the skins with a small sharp knife and roughly chop.
Heat the butter in a saucepan, add the onion and fry gently for 5 minutes until just beginning to turn golden around the edges. Dice 4 rashers of the bacon and add to the pan along with the potato and chestnuts. Fry gently for 5 minutes, stirring occasionally.
Add the brandy and, when bubbling, flame with a long taper and quickly stand back. As soon as the flames have subsided pour in the stock. Add the thyme, spices and seasoning and bring to the boil, cover and simmer for 45 minutes.
Discard the thyme sprig and purée half the soup in a blender or food processor until smooth. Return to the soup in the pan and reheat. Taste and adjust the seasoning if needed. Wrap each remaining rasher of bacon around a skewer and grill until crisp. Ladle the soup into cups and top with the bacon skewers. Drizzle with the truffle oil and a little extra brandy if liked.