Brown the ground beef in a dutch oven or stock pot until no longer pink, adding salt to season. Drain all the fat. Add the bacon fat and continue cooking, add onion and cook until softened. Add the sweet potato and the serrano chili and cook about five more minutes. Add the tomato paste, chili powders, cumin, a little more salt, black pepper and allspice and brown for another five minutes. Slowly add the beef stock, cocoa powder, chopped tomatoes and water. Mix all together and bring to a simmer. Cover the pot and simmer for at least an hour until thickened and slightly reduced. I simmer this most of the day very low, like maybe three hours. You could also throw it all in the slow cooker on low and simmer all day.