Start by preparing the chicken. Place a whole chicken in a large pot and cover it with cold water.
Bring the water to a boil, then reduce the heat and let the chicken simmer for 60-90 minutes, until the meat is cooked through.
Remove the chicken from the pot and set it aside to cool. Once it's cool enough to handle, shred or chop the meat into bite-sized pieces.
In the same pot, sauté the leeks in a bit of butter or olive oil until they're soft and fragrant, about 5-7 minutes.
Add the shredded chicken back to the pot, along with chicken broth, rice, thyme, bay leaves, and black pepper. Bring the soup to a simmer and let it cook for 20-30 minutes, until the rice is tender.
In the final 5 minutes of cooking, stir in the prunes to allow them to heat through.
Taste the Scottish soup and adjust the seasoning as needed, adding more salt, pepper, or herbs to your preference.
Ladle the hot cock a leekie soup into bowls and serve immediately, garnished with fresh parsley if desired.