Mix the soy, mirin, water and caster sugar together in a saucepan. Cook for 5 mins until it has reduced by a quarter.
Mix the cornflour with a little water and pour this milky paste into the sauce. Whisk well and cook for a further few minutes until you have a thick glossy sauce.
Preheat the grill to high and line a large flat baking tray with foil.
Cut 4 of the spring onions into four pieces. Thinly slice the remaining spring onion. Cut the chicken into large chunks, around an inch big. Thread the chicken onto the skewers, alternating with pieces of spring onion until the skewers are full. Brush the skewers all over with the yakitori sauce.
Place the skewers on a baking tray and place under the grill. Cook for 15 minutes, turning regularly and basting every 3 minutes with the sauce. To serve, scatter over the remaining spring onions and sesame seeds. Eat whilst it’s hot and enjoy.