Prepare Marinade: In a mixing bowl, whisk together soy sauce, brown sugar, pineapple juice, ketchup, vegetable oil, minced ginger, minced garlic, rice vinegar, and black pepper until well combined and glossy, about 2-3 minutes.

Marinate Chicken: Place the boneless, skinless chicken thighs into a large resealable bag. Pour the marinade over the chicken, ensuring each piece is well coated. Seal the bag tightly and refrigerate for at least 1 hour, or overnight.

Preheat Grill: About 15 minutes prior to grilling, preheat your grill to medium-high heat (around 375°F). Ensure it's clean and lightly oiled.

Remove Chicken: Once marinated, remove the chicken from the fridge and discard the leftover marinade.

Grill Chicken: Grill chicken thighs for 6-8 minutes on each side until the internal temperature reaches 165°F and they are charred.

Baste (Optional): Brush the chicken with reserved marinade during the last few minutes of grilling.

Rest Chicken: Let the grilled chicken rest on a cutting board for about 5 minutes before serving.

Garnish: Sprinkle with chopped green onions and sesame seeds before serving.