Place the butter, bacon, onions, shallot, and garlic in a large casserole and dust them with the flour. Add the salt and pepper and the herbs. Make a bed of sliced carrots and place the ducks upon it. Moisten them with the beef stock and simmer gently with the casserole covered for about an hour and a half. During the last 20 minutes of cooking add the cognac and green olives. Arrange the ducks on a platter surrounded by the carrots and olives. Stir the sauce until thick and smooth and pour it over the birds.