1. Place the beans in a large, heavy-bottomed pot with 6 cups of water. Remove and discard any floating beans or debris.

2. Turn the heat to high and bring the water to a rapid boil. Cook for 2 minutes, then remove from the heat. Cover the pan with a lid and let sit for 1 hour.

3. Pour the beans into a strainer and rinse off the excess starch.

4. Heat the butter and olive oil in a large soup pot over medium heat. Add the onions and carrots and cook until soft. Add the celery and garlic and cook 2 minutes more.

5. Add the black beans, chili, cumin, bay leaf, salt, pepper and ham hock.

6. Pour in the remaining 6 cups of water, mix well, and bring the soup to a boil.

7. Reduce the heat to a lively simmer, cover and simmer for approximately 1 ½ hours, or until the beans are cooked.

8. Remove the soup from the heat and let sit for 15 minutes to cool.

9. Remove the ham hock and strip it of all visible fat. Shred the meat and set aside.

10. Measure out about half the beans and 1 ½ cups of the liquid. Puree this mixture using a blender, food processor, food mill or emersion blender.

11. Return the puree back to the pot and mix well.

12. Stir in the sherry and shredded meat.

13. Return the soup to a medium-high flame and bring to a simmer.

14. Ladle into bowls and garnish with sprigs of cilantro.