Cover the pork with cold water in a pot and bring to the boil. When the scum rises to the top, drain the pork, rinse it under fresh cold water and pat dry with paper towel.
Heat a wok until just smoking. Add the peanut oil to the wok and, when hot, add the dried chillies, garlic and ginger and stir-fry until fragrant. Add the bean paste and stir-fry for a minute more. Add the pork and gently stir-fry for a further 2 minutes. Deglaze the wok with the shaoxing, and add the soy sauce, salt, sugar and cassia bark. Cover with chicken stock. Bring to the boil, then lower to a gentle simmer, simmering uncovered for about 1½ hours, or until tender.
Reheat the noodles in boiling water and drain. Place them in a large bowl and spoon the pork over. If the sauce is not quite thick enough, reduce over high heat for a minute or two, then pour the sauce over the noodles and sprinkle with spring onion. Serve with your favourite chilli sauce.