In a bowl, mix soy sauce, mirin, sugar, and sake (if using). Set aside.
Heat oil in a pan over medium heat. Add chicken and cook until browned on all sides.
Pour in the sauce mixture and bring to a simmer.
Cook for 5β7 minutes until the sauce reduces slightly.
Optional: To thicken the sauce, mix cornstarch and water in a small bowl, then stir into the pan and cook until thickened.
Garnish with sesame seeds and chopped green onions before serving.
Serve hot over rice or noodles.