Mix together the venison, wine, thyme, garlic, star anise and Cognac in a large bowl. Cover with cling film (plastic wrap) and leave to marinate in the refrigerator for at least three hours. Drain the venison through a colander into a bowl, reserving the flavoured marinade.
Heat 4 tablespoons of the oil in a large, flameproof casserole dish over a medium heat. Add the venison and cook for at least 20 minutes until very well coloured on all sides but not burnt. Season with salt and pepper, then sprinkle in the flour and cook, stirring, for a further 2–3 minutes until the meat is lightly coated. Add the stock and the reserved marinade and bring to the boil, skimming off any foam that rises to the surface. Add the bouquet garni, carrots and onions, turn the heat down to low, partially cover and simmer for 1 hour 50 minutes, stirring occasionally, until the meat is very tender and the sauce is rich and silky.
Five minutes before the venison is ready, remove 2 ladles of the sauce into a bowl. Whisk in the chocolate until melted and combined. Return the chocolate flavoured sauce to the casserole dish, stir together well, then cover it with a lid and turn off the heat.
Heat the remaining oil in a frying pan (skillet) over a medium heat. Add the pancetta and fry for 2–3 minutes until just coloured, then stir in the wild mushrooms and fry for a further 1–2 minutes until just tender. Add to the venison and stir through. Season with salt and pepper to taste, then stir in the parsley, taking care not to break up the venison, and serve.