Mix together the tomato purée and stock in a jug. Place the pork in a large saucepan and cover with the stock. Add the cinnamon stick and bring to the boil, then reduce the heat and simmer for 1½ hours, or until the pork is cooked through. Remove the pork from the stock, reserving the liquid. Use a fork to shred the meat.
Meanwhile, heat a nonstick frying pan until hot, add the chillies and dry-fry for 1 minute on each side until they start to crackle. Remove the pan from the heat, pour in boiling water to cover and leave to soak for 30 minutes.
Cook the onion and tomatoes under a preheated hot grill for 7 minutes. Turn them over and add the garlic, then cook for a further 5–7 minutes until softened and lightly charred. Transfer to a food processor or blender.
Heat 2–3 tablespoons of the lard or oil in a frying pan, add the almonds and cook for 3 minutes until lightly browned, then tip into the blender. Add the raisins to the pan and cook for 1 minute, or until puffed. Drain the chillies and add to the blender with the raisins and 200 ml (7 fl oz) of the pork cooking liquid. Whizz until smooth.
Add a little more lard to the frying pan, pour in the tomato sauce and cook for 5 minutes, stirring continuously, until thickened. Add the cumin, oregano and chocolate, top up with more cooking liquid, then simmer for 30 minutes. Stir the pork into the sauce and cook for a further 5 minutes. Serve with warm tortillas and salsa (see below).