Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the beef and onion and fry, stirring, until the mince is evenly browned.
Stir in the garlic, ground spices and bay leaf and cook for 1 minute. Mix in the tomatoes, beans, sugar and stock, then add the chocolate and a little salt and pepper and bring to the boil, stirring.
Pour into the slow cooker pot, cover with the lid and cook on low for 8–10 hours or until cooked through. Stir, spoon on to plates and top with a little soured cream, salsa (see below) and extra paprika. Serve with rice.