Melt the butter in a large pot over medium heat. Stir in the carrots, celery, and onion, and cook 15 minutes. Pour in chicken broth. Mix in diced tomatoes with green chile peppers, black beans, sherry, and garlic. Season with cumin, cayenne pepper, and salt. Bring to a boil, reduce heat to low, and simmer 20 minutes. Mix in cilantro during last few minutes of cook time.
