Unpack the chestnuts and break them apart. Melt the butter in the saucepan and stew the chestnuts for about 10 minutes. Top up with the vegetable stock. Add the cream and simmer gently for another 10 minutes. Season to taste with salt and pepper. If you like, you can thicken with a little cornstarch, but since some chestnuts always disintegrate when steaming, it is actually creamy enough.