Combine mince and onion in a large saucepan over medium-high heat. Cook, stirring, until beef is browned, about 5 minutes.
Transfer cooked beef and onions to slow cooker. Stir in water, tomatoes, kidney beans, black beans, corn, chocolate chips, chilli powder, cumin, oregano and salt. Cook on High until chilli begins to bubble, about 20 minutes. Reduce heat to Low and cook until thick, about 2 hours.