Put the wood ear and shiitake mushrooms in a bowl, cover with just boiled water and soak for 20 minutes. Drain, keeping 100 ml of the soaking liquid. Slice both thinly, discarding any tough shiitake stems.

Bring the stock and the reserved soaking liquid to a simmer in a large pan. Add the mushrooms, bamboo shoots and pork, if using, and simmer for 5 minutes.

Add the tofu, light soy sauce, dark soy sauce, sugar and white pepper. Simmer for 3 minutes more.

Mix the cornflour with the cold water until smooth. Pour it into the pan in a thin stream while stirring, then simmer for 2 minutes until the soup thickens and turns glossy.

Turn the heat down so the soup barely moves. Pour the beaten egg in a slow, thin stream while stirring in one direction, so it sets into ribbons rather than lumps.

Take the pan off the heat. Stir in the black vinegar and sesame oil now, not earlier, so the sharpness stays bright.

Taste and adjust: more vinegar for sour, more white pepper for heat. Serve topped with spring onions and a spoon of chilli oil.