Place 1 tablespoon of the cornflour in a large bowl and stir in the rice wine to make a paste, then stir in the soy sauce, sesame oil, egg yolk and salt. Add the pork and mix well, then cover and leave to marinate for 2 hours or up to overnight in the refrigerator.
Stir the ingredients for the sweet and sour sauce in a pan and bring to the boil. Add the peppers and carrot and reduce the heat. Simmer gently for 5 minutes, stir in the pineapple and cook for a further 3–4 minutes until the vegetables and fruit are tender. Stir in the soy sauce and spring onions and set aside.
Combine the flour and remaining 3 tablespoons of cornflour and stir into the marinated pork.
Pour enough oil into the wok to deep-fry the pork, and heat it to 190°C (375°F), or until a cube of bread dropped into the oil turns golden in 20 seconds. Deep-fry the pork in batches for 3–4 minutes, until golden. Remove using a slotted spoon and drain on kitchen paper, then toss into the sweet and sour sauce. Serve with rice, if liked.