Heat olive oil in a frying pan over medium heat and sauté onion, garlic, and celery until softened.
Add mushrooms and chillies to the pan and cook for one minute.
Stir in tomato purée and cook for an additional 2 to 3 minutes.
Incorporate cocoa powder and 150ml of vegetable stock then bring the mixture to a boil.
Simmer on low heat for 15 minutes.
Add beans and cook for 10 to 12 minutes until the sauce reduces and beans are heated.
Fill each taco shell with 2 to 3 tablespoons of the bean chilli.
Garnish with grated cheese, chopped coriander, and a dollop of yoghurt before serving.