Step 1: In a large skillet or Dutch oven, brown the ground beef over medium-high heat. Break it up with a spoon as it cooks. Once fully browned, drain off any excess fat. Transfer the browned beef to a 6-quart (or larger) slow cooker.
Step 2: Add the chopped onion and bell pepper to the same skillet. Sauté for 5-7 minutes until softened. Stir in the minced garlic and cook for another minute until fragrant.
Step 3: Add the chili powder, cumin, smoked paprika, oregano, and cayenne pepper (if using) to the skillet with the vegetables. Cook for 1 minute, stirring constantly, until the spices are fragrant.
Step 4: Transfer the sautéed vegetable and spice mixture to the slow cooker with the beef. Add the diced tomatoes (undrained), tomato paste, beef broth, drained and rinsed kidney beans, and pinto beans. Stir everything together until well combined.
Step 5: Cover the slow cooker and cook on LOW for 6-8 hours or on HIGH for 3-4 hours. The longer cook time on LOW will result in deeper flavors.
Step 6: About 30 minutes before serving, stir in the chopped unsweetened chocolate (if using) and apple cider vinegar. Season with salt and black pepper to taste. Allow to cook for the remaining time to let the flavors meld. Serve hot with your favorite toppings.