Preheat the oven to 190°C, 170°C fan, 375°F, gas 5. Start by marinating the cubed chicken in the soy sauce, sugar, rice wine vinegar, ginger and garlic in the fridge for 30 minutes, or up to 8 hours, if you have the time. Meanwhile, wash the rice in a sieve under plenty of cool water, then leave to drain for 30 minutes. Put the rice in a pan with enough water to cover it by 2cm. Cover with a lid and bring to the boil. Simmer gently for 12 minutes. Combine the rice wine vinegar, caster sugar and salt then stir into the hot rice. Leave to cool completely.
Put the cornflour into a shallow bowl, the eggs into another shallow bowl and spread the breadcrumbs out on a large plate. Line a large baking sheet with non-stick baking paper. Drain the excess marinade from the chicken. Working in small batches, dust the chicken all over with the cornflour, then dip it in the beaten egg, then coat completely with the breadcrumbs, pressing them in firmly. Spread the coated chicken out on the baking sheet. Drizzle with the sesame or sunflower oil. Bake for 18-20 minutes, until golden and crisp. Check the meat is cooked through with no pink showing.
Roll the sushi rice into balls the size of a small walnut. You can also roll them in toasted sesame seeds to coat, if you like. Serve with the chicken nuggets and plum sauce, hoisin sauce or ketchup, to dip. To keep the chicken and rice cool in a lunchbox, pop a frozen water or juice bottle into the box in the morning. It will keep the lunchbox cool for a few hours as it defrosts.
Tip: Coating the chicken with cornflour, then egg, then dried breadcrumbs before baking ensures they will stay crisp for a few hours. Rolling the cooked rice into balls takes away the need for cutlery and makes it easier for children to eat. Leave out the ginger and garlic in the chicken marinade if you want to simplify things.
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For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.