In a medium bowl, whisk together the soy sauce, brown sugar, pineapple juice, ketchup, vegetable oil, minced ginger, garlic, rice vinegar, and black pepper until the brown sugar dissolves completely and the marinade is smooth.

Optional: Pour 1/4 cup of the marinade into a small jar and refrigerate it for later use as a serving sauce. This reserved portion is safe to use without boiling since it hasn't touched raw chicken.

Place the chicken thighs in a large resealable plastic bag or shallow dish. Pour the remaining marinade over the chicken, making sure every piece is thoroughly coated. Seal the bag or cover the dish.

Refrigerate for at least 1 hour. For best results, marinate for 4 hours or overnight. Do not marinate longer than 24 hours as the pineapple enzymes can over-tenderize the meat.

Preheat your grill to medium-high heat with a grill surface temperature around 375 to 400°F. Clean and oil the grill grates well to prevent sticking.

Remove the chicken from the marinade, allowing excess liquid to drip off. Discard the used marinade.

Place the chicken thighs on the grill and cook for 6 to 8 minutes on the first side without moving them, watching for flare-ups from the sugar in the marinade.

Flip the chicken and cook for another 6 to 8 minutes on the second side until the internal temperature reaches 165°F in the thickest part and the outside has a caramelized glaze with grill marks.

Optional: If you reserved clean marinade in step 2, brush it on during the last 2 minutes of cooking for extra glaze, or simmer it in a saucepan for 5 minutes and use as a serving sauce.

Remove the chicken from the grill and let it rest for 5 minutes on a clean plate before slicing.

Garnish with chopped green onions and sesame seeds if desired before serving.