1. Heat the oil in a large pot over medium-low heat. Add the onion, carrot, celery, garlic and sweet potato; cook 8 to 10 minutes, until tender. Season the pork with the salt and black pepper; add the meat to pot and cook until browned, stirring frequently, about 5 minutes.
2. Mix in the chili powder, cumin, dark-chocolate powder, oregano and cinnamon; cook 2 minutes. Add the vegetable stock, tomatoes, chipotle pepper and adobo sauce; stir, then bring to boil. Reduce heat to low; cover pot and simmer, stirring occasionally, until pork is tender, about 1 hour. Stir in the navy beans and cilantro; cook until heated through, about 3 minutes. Serve the chili topped with more cilantro and a dollop of yogurt if desired.