In a bowl, combine the diced chicken with the Japanese soy sauce, sake, mirin and caster sugar. Mix well and marinate for 15–20 minutes.
Heat the vegetable or peanut oil in a wok over high heat. Add the marinated chicken and stir-fry for about 4 minutes until nearly cooked through.
Add the diced carrot, mangetout, spring onions and sweetcorn. Stir-fry for another 2 minutes until the vegetables are just tender.
Push everything to one side of the wok. Pour the beaten egg into the empty space and gently scramble until just beginning to set.
Add the cooled sushi rice and stir-fry everything together for 1 minute, breaking up any clumps of rice.
Add the Japanese soy sauce and a pinch of salt. Toss well until the rice is evenly coated and heated through.
Garnish with the reserved spring onions and serve immediately.