Finely chop the onions and the garlic, and then dice the celery and the red pepper.
Heat up a large, heavy bottomed saucepan to a medium temperature and add the olive oil. Add the chopped onions and celery and fry for a few minutes, before adding the diced red pepper and the chopped garlic, and frying for a few more minutes until the vegetables are softening.
Add the minced beef to the pan, season it well with salt and pepper, and then brown it off for five minutes.
Add to the pan the smoked paprika, the ground cumin, the dried oregano, the chili powder, and the ground coriander, and then stir the herbs and spices in, letting them cook for a minute or two until fragrant.
Add the cans of chopped tomatoes to the pan along with the tomato purée and thoroughly mix these into the beef and vegetables, then pour in the beef stock. Reduce the pan to a simmer and allow the mixture to cook for at least 45 minutes, stirring occasionally. If the mixture gets too dry, add a dash of water.
When the chili is thick and fragrant, turn off the heat and stir in the dark chocolate and a dash or two of Worcestershire sauce. Serve sprinkled with fresh cilantro if desired.