Combine the pork, soy sauce and salt in a small bowl and mix well. Heat a wok or sauté pan until it is hot. Add the oil and deep-fry the pork, stirring with a spatula to break it into small pieces (Caution: hot oil can be dangerous. Do not leave unattended.) When the pork is crispy and dry, after about 5-6 minutes, remove it with a slotted spoon and drain on a plate lined with kitchen paper.
Pour off the oil and strain and reserve it. Pour two tablespoons of the oil back in the wok. Reheat the wok and add the garlic, ginger and spring onions, and stir-fry for 30 seconds. Add the sesame paste (or peanut butter), soy sauce, chilli oil, salt and chicken stock and simmer for four minutes.
Cook the noodles in a large pan of boiling water for two minutes if they are fresh, or five minutes if they are dried. Drain the noodles well in a colander. Divide the noodles into individual bowls or put them in a large soup tureen. Ladle on the sauce, garnish with the fried pork, Sichuan peppercorns and chopped chilli, if using, and serve at once.