Place the butternut squash pieces into a roasting tin and drizzle over three tablespoons of the olive oil. Transfer to the oven and roast for 25-30 minutes, or until the squash is tender and golden-brown. Set aside to cool.
When the butternut squash has cooled, drain off the oil and peel away the skin, taking care not to burn your fingers.
Heat the remaining olive oil in a large pan over a medium heat, add the onion, garlic, potatoes and apple and fry for 4-5 minutes, stirring regularly, until softened.
Add the stock, roasted squash, thyme, bay leaves, chestnuts and the chestnut purée. Stir well and season, to taste, with salt and freshly ground black pepper.
Bring the mixture to the boil, turn down the heat and simmer for 15-20 minutes, or until the potatoes are tender. Set aside to cool.
Transfer the mixture to a food processor and blend until smooth. Return to the pan and heat until warmed through.
To serve, divide the soup equally between ten serving bowls. Garnish each serving with a few drops of truffle oil and cream (if using) and sprinkle over the sliced roasted chestnuts.