Combine soy sauce, honey, vinegar, Chinese cooking wine, sesame oil, and garlic in a jug. Stir well until honey is fully dissolved and the sauce is smooth.

Before adding chicken: measure out 1/4 cup for basting and 2/3 cup for the serving sauce. Refrigerate both portions separately right away.

Pour remaining marinade over chicken, coat thoroughly, cover, and refrigerate for 24 to 48 hours (minimum 3 hours).

To make the serving sauce: pour the reserved 2/3 cup into a small saucepan. Whisk cornstarch into the 1/3 cup water until smooth, stir into saucepan, and simmer over medium heat for 4 to 5 minutes until thick and syrupy. Keep warm on low heat.

Heat grill or skillet to medium-high and brush with vegetable oil.

For chicken thighs: cook first side 2 minutes, flip and baste with reserved clean basting sauce. Continue flipping and basting every 1 minute for a total cook time of about 10 minutes, or until internal temperature reaches 165 F.

For chicken breasts: cook first side 2 minutes, flip and baste every 45 seconds for a total cook time of about 8 minutes, or until internal temperature reaches 151 F.

Transfer chicken to a plate, cover loosely with foil, and rest for 5 minutes before serving.

Serve garnished with sesame seeds and sliced green onion, with the warm serving sauce on the side.