For a spice bag, cut a 4-inch square from a double thickness of 100-percent-cotton cheesecloth. Place bay leaf, allspice, and cloves in the center of the cheesecloth square. Bring up corners of cheesecloth, tie closed with 100-percent-cotton kitchen string, and set aside.
In a large skillet cook ground beef over medium heat until browned, using a wooden spoon to break up meat as it cooks. Drain off fat. Transfer meat to a 3-1/2- or 4-quart slow cooker. Stir onions, beans, tomato sauce, the water, chili powder, garlic, Worcestershire sauce, cumin, cinnamon, salt, and cayenne pepper into meat in cooker. Stir in spice bag.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours, stirring in chocolate during the last 30 minutes of cooking.
To serve, remove spice bag. Spoon chili over hot cooked spaghetti. Sprinkle each serving with cheese.