Whisk broth, tomato sauce, tomato paste, vinegar, chili powder, cocoa powder, cumin, cinnamon, cloves, allspice, salt and pepper in the base of a slow cooker. Add garlic and yellow onion, and crumble in beef; mix. Cover and cook on HIGH for 6 hours or LOW for 8 hours.
Divide cooked spaghetti into 8 bowls. Top with chili and, if desired, finely shredded cheddar and diced white onions.