Heat the oil in a large, deep pan or saucepan over a medium heat. Add the mince, and fry for 5-10 mins, or until browned all over. Use a wooden spoon to break up the chunks as you go.
Reduce the heat to medium low and add the onion, garlic, chilli powder, paprika, cumin, brown sugar and plenty of salt and pepper.
Cook gently for a further 10 minutes, stirring frequently so the spices are toasted in the pan. Stir and reduce the heat if the mixture starts to 'catch' on the bottom of the pan.
Add the tomatoes, tomato puree, kidney beans, red peppers and beef stock, stirring well, pop a lid on the pan.
Simmer gently for 45 minutes.
Remove from the heat, stir through the chocolate until it's melted. Check for seasonings. If it's not spicy enough add more chilli powder to taste.