Pour 100ml of boiling water over the dried porcini and leave to stand for 5 mins. In a saucepan, heat the butter and add the shallots, frying gently until they are soft and translucent. Add the chestnut mushrooms and cook for 5 mins more. When they are cooked, pour in 1 tbsp of the liquor from the porcini and discard the rest. Chop the porcini and add them to the pan. Fold in the crème fraîche, bring to a simmer then season.