Cut the chicken into bite-sized pieces.
In a bowl, mix soy sauce, mirin, brown sugar, sesame oil, garlic, and ginger.
Marinate the chicken in the mixture for at least 15 minutes.
Heat a pan over medium-high heat and add a little oil.
Add the marinated chicken to the pan, reserving the marinade.
Cook the chicken until browned and cooked through, about 6-8 minutes.
Pour the reserved marinade into the pan and bring to a simmer.
Mix cornstarch with water and add it to the pan to thicken the sauce.
Cook for another 2-3 minutes until the sauce is glossy.
Serve hot, garnished with sesame seeds and green onions.