Cut the chicken into small pieces and put in a bowl.
Mix the soy sauce, brown sugar, mirin and vinegar for the teriyaki sauce.
Pour half of the teriyaki sauce over the chicken pieces. Reserve the remaining teriyaki sauce.
Leave the chicken to marinate in the teriyaki sauce for 30 minutes at room temperature.
Heat the oil in a non-stick frying fan over medium-high heat. Put the chicken pieces on the frying pan and cook, stirring periodically, for 5 minutes or until cooked through (no longer pink in center).
Mix the remaining teryaki sauce with the cornstarch. Add this sauce to the frying pan and mix with the chicken. Keep stirring for 2 minutes, or until the sauce has thickened.