Cook the soba noodles according to the packet instructions. Rinse in a colander under hot running water. Drain well.
Heat the sesame oil in a large wok. Add the spring onions and pak choi, and stir-fry over medium-high heat for 1 minute. Pour in the stock, sake and soy sauce and simmer gently for 5 minutes. Stir in the bean sprouts.
Make the tempura while the stir-fry is simmering. In a bowl, briefly whisk together the egg yolk, flour and water to make a slightly lumpy batter. Heat the vegetable oil in a separate wok, a deep heavy-based saucepan or a deep-fat fryer to 180°–190°C (350°–375°F), or until a cube of bread dropped into the oil browns in 30 seconds. Dip the prawns in the batter, then carefully drop them into the hot oil and cook for 3 minutes or until the prawns have turned pink and the batter is golden. Remove with a slotted spoon and drain on kitchen paper.
Spoon the noodles into warm soup bowls, add the soup and top with the prawns and strips of nori, if liked. Serve immediately.