Coat slow-cooker bowl with nonstick cooking spray.
Layer sweet potatoes, onion, green pepper, garlic and pinto beans in slow cooker. Combine tomatoes, chili powder, cocoa, cumin, salt, cayenne and 2 cups water in a medium-size bowl. Pour over vegetables.
Cover and cook on HIGH for 6 hours or LOW for 8 hours.
Stir cilantro into chili and serve over brown rice.
