Brown ground beef in a large skillet over medium-high heat, breaking it into small pieces. Drain fat and transfer to slow cooker.
Add onion, garlic, tomato sauce, tomato paste, beef broth, and apple cider vinegar to the slow cooker.
Stir in all the spices: chili powder, cocoa powder, cinnamon, cumin, paprika, allspice, cayenne pepper, and salt. Add bay leaves.
Mix everything together until well combined. The mixture will be fairly thin—that's exactly what you want!
Cover and cook on LOW for 5-6 hours or HIGH for 3 hours. Remove bay leaves before serving.
To serve Cincinnati-style: Place a portion of cooked spaghetti in each bowl, ladle chili on top, and pile high with shredded cheddar cheese. Add diced onions and warm kidney beans if you want to make it a "five-way"!