Heat the oil in a casserole dish over a medium heat, and fry the onion and peppers for 10-12 mins, or until softened. Add the garlic, coriander stalks, ground coriander, cumin and chipotle paste, and fry for 2 mins more. Tip in the tomatoes, tomato pureé and stock, then add the cinnamon stick. Nestle the chicken thighs into the mixture, then reduce the heat to low and simmer, covered, for 45 mins or until the chicken is tender. Remove the chicken from the casserole and transfer to a chopping board, then shred the meat off the bone using two forks.
Return the shredded chicken to the pan, then tip in the black beans along with the liquid from the can, the drained kidney beans, vinegar and chocolate. Stir well to combine all of the ingredients and melt the chocolate, then turn the heat up to medium and simmer, uncovered, for 10-15 mins, or until the mixture has thickened slightly.
Remove the cinnamon stick, season to taste and stir though most, or all of the coriander leaves (you can reserve a few leaves to scatter over at the end, if you like). Serve the chilli with rice or tortilla chips, topped with guacamole and soured cream, if using, and scatter over any remaining coriander.