In a bowl, combine soy sauce, mirin, sake, sugar, garlic, and ginger. Stir until sugar is dissolved.
Place chicken thighs in a resealable plastic bag or a shallow dish and pour the marinade over chicken. Marinate for at least 30 minutes.
Heat vegetable oil in a large skillet over medium heat. Add chicken thighs and cook until browned on both sides, about 5-7 minutes per side.
Pour the marinade into the skillet and bring it to a boil. Reduce heat and let it simmer until the chicken is cooked through and the sauce has thickened.
If the sauce is too thin, add the cornstarch mixed with water and stir until the sauce has thickened.
Serve the teriyaki chicken over steamed rice and garnish with chopped green onions or sesame seeds if desired.