In a large skillet over medium-high heat, cook the ground beef and diced onion until the beef is fully browned and no longer pink. Drain off any excess fat.
Transfer the cooked beef and onions into the insert of your slow cooker.
Add the minced garlic, kidney beans, pinto beans, diced tomatoes, tomato sauce, chili powder, cumin, dark chocolate, and Worcestershire sauce to the slow cooker. Stir well until all ingredients are fully combined.
Place the lid on the slow cooker. Cook on LOW for 6 to 8 hours, or on HIGH for 3 to 4 hours.
Ladle the warm chili into bowls. Garnish generously with sour cream, sliced green onions, and fresh cilantro. Serve immediately with tortilla chips on the side for dipping.