In a mixing bowl, whisk together pineapple juice, soy sauce, brown sugar, ketchup, rice vinegar, garlic, ginger, and sesame oil (if using) to make the marinade.

Place the chicken in a large resealable plastic bag or shallow dish. Pour the marinade over the chicken, ensuring all pieces are fully coated. Seal the bag or cover the dish, then refrigerate for at least 4 hours, or preferably overnight for deeper flavor.

Preheat your grill to medium-high heat. If using a charcoal grill, set up a two-zone fire with direct and indirect heat. Lightly oil the grates to prevent sticking.

Remove the chicken from the marinade, reserving the leftover marinade for basting. Place the chicken on the grill and cook for 5–7 minutes per side, basting occasionally with the reserved marinade, until the internal temperature reaches 165°F (74°C).

(Optional) To create a glaze, pour the reserved marinade into a small saucepan. Bring it to a boil, then reduce the heat and simmer for 5–10 minutes. Mix cornstarch with water to create a slurry, stir it into the sauce, and cook until thickened.

Once cooked, remove the chicken from the grill and let it rest for a few minutes. Garnish with sliced green onions and toasted sesame seeds if desired.

Serve hot with steamed rice, grilled pineapple slices, or your favorite sides. Enjoy!