Put the porcini mushrooms in a bowl and pour boiling water over them. Leave to soak for 10 mins.
Melt the butter in a frying pan until foaming and fry the shallots over a low heat for 10 mins or until softened and translucent. Add the chestnut mushrooms and fry for 8 mins. Drain (freeze the liquid for stock or risotto) and finely chop the porcini mushrooms, then add to the pan, frying for another 5 mins. Add the garlic and cook for 1 min. Set aside to cool for 20 mins.
Tip the mushroom mixture into a food processor, along with the mascarpone, vinegar, parmesan and seasoning. Blitz until smooth while drizzling in the truffle oil. Divide between six ramekins.
For the topping, melt the butter in a small pan. When the milk solids separate and sink to the bottom, pour the clarified butter into a jug. Mix with the truffle oil and pour over each ramekin. Sprinkle with the thyme and peppercorns. Chill for at least 5 hrs. Serve with cornichons and toast, if you like.