Mix soy sauce, mirin, sugar, sake (if using), garlic, and ginger in a bowl to make the teriyaki sauce. Set aside.

Heat vegetable oil in a large skillet over medium heat. Add chicken and cook for 4-5 minutes on each side until browned and fully cooked. Remove and set aside.

In the same skillet, pour the prepared teriyaki sauce and bring it to a simmer.

Return the chicken to the skillet and cook for another 2-3 minutes, turning occasionally to coat the chicken evenly with the sauce.

Slice the chicken and serve hot with steamed rice and vegetables, drizzling extra sauce on top.