1. Melt the butter in a large pot and fry the chicken on all sides, turning occasionally until lightly brown all over.
2. Roughly chop the white part of the leeks (reserve the green tips for later) and add to the pot. If you're using the bacon, add now. Fry for about 5 minutes, until the leeks are tender.
3. Pour in the chicken stock and add the bouqet garni. Finely chop the remaining green parts of the leek and add these too.
4. Cover and simmer for 1 hour, topping up with water or more stock if necessary.
5. Season the soup to taste and add the prunes. Cover and simmer for a further 30 minutes.
6. Remove the chicken, strip the meat from the carcass and chop into pieces. Skim the soup if necessary. To serve, place the chicken pieces in bowls and pour over the soup. Garnish with the parsley.