Stir together gochujang, soy sauce, mirin, honey, garlic, sesame oil, ginger, water and pepper in a bowl, add the chicken and marinate for at 2 hours.
Heat oil over medium-high heat in a wok or large frying pan. Turn the heat down to medium ,add the chicken to the pan, reserving the remaining marinate and cook the chicken until no longer pink, about 10 minutes. Add the remaining remaining and stir.
Drizzle in the cornflour and stir together until a little thickened.
Top with sesame seeds and green onions. Serve immediately with rice.
