ADD the cornstarch to a small bowl. SLOWLY add 4 tablespoons of the soy sauce, mixing constantly, until the cornstarch has dissolved. STIR in the sugar and 2 tablespoons of the mirin. HEAT the sauce mixture in a saucepan over a high heat, stirring constantly, until the mixture thickens. PREHEAT the oven to 450°F/230°C. PLACE the chicken in a baking tray lined with baking paper and spoon the teriyaki sauce on top of each fillet (use the back of the spoon to spread the sauce out evenly). BAKE for 20 minutes. WHILE the chicken bakes, cook the soba noodles according to package directions then toss with the sesame oil and remaining tablespoon of soy sauce and ½ tablespoon of mirin. REMOVE the chicken from the oven, allow to rest for 5 minutes, then thickly slice. SERVE the chicken and noodles in large bowls with the corn, edamame, carrot, cucumber, red pepper, radishes and scallion.