To prepare the teriyaki sauce: put the soy sauce , mirin, and superfine sugar into a small saucepan and gently cook for 20 minutes. Skim the surface of any scum and set aside
Cut the chicken into 1 inch-square pieces. Wash the leeks, dry and then cut into 1 3/4-inch lengths. Soak the bamboo skewers in water for a few minutes to stop them from burning during cooking. You may need to trim off 2-3 inches of the skewers so they fit in the skillet
Thread the chicken and leeks onto the skewers, alternating between the chicken and the leek pieces
Put a little oil into a large skillet and when well-heated, add the skewers and fry until nicely browned and cooked through. You can also cook these skewers on a barbecue or simply broil them
When the chicken and leek skewers are ready to serve, coat them in the teriyaki sauce or alternatively sprinkle with salt and pepper. Arrange on a large plate and serve