Heat the oil in a frying pan and cook the onion on a medium heat until softened. Add the beef with some black pepper and a little salt, breaking up any lumps with a wooden spoon, and cook until browned. Add the chipotle and Cajun seasoning. Give it a good stir and cook for 1 min more.
Tip in the beans and tomatoes, then crumble over the stock cube. Add a can of water and simmer for about 20-30 mins, until thickened. Add the chocolate, and stir until melted, then add most of the coriander.
Serve the chilli in bowls on top of the rice sprinkled with the rest of the coriander, with a dollop of low-fat yogurt, and an extra drizzle of chipotle paste on top.