Heat a medium casserole pan over a high heat, and when hot, turn the heat down to medium and add the butter. When the butter is foaming, add the onion, celery, star anise and salt. Cook for 10 mins until softened, then stir in the garlic, chipotle, oregano and ancho chilli flakes. Cook for 2-3 mins to soften the garlic before stirring in the black beans.