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

Coat the boneless, skinless chicken thighs generously in the marinade. Cover and refrigerate for at least 1 hour, or overnight for enhanced flavor and tenderness.

While the chicken marinates, prepare your grill by preheating it to medium-high heat.

When ready to grill, take the chicken out of the marinade, letting any excess drip off. Discard the leftover marinade.

Place the chicken thighs on the grill and cook for 6-8 minutes per side, ensuring the internal temperature reaches 165°F (75°C).

If desired, brush the chicken with some reserved marinade during the last few minutes of grilling.

After grilling, let the chicken rest for about 5 minutes before serving.

Sprinkle freshly chopped green onions and optional sesame seeds over the chicken to add color and crunch.