In a large bowl, whisk together pineapple juice, soy sauce, brown sugar, rice vinegar, grated ginger, minced garlic, ketchup, sesame oil, and black pepper until well combined.

Pour 1/2 cup of the marinade into a separate small bowl. This portion will be used for basting later and will not come into contact with raw chicken. Cover and refrigerate it.

Add the chicken thighs to the remaining marinade in the large bowl, ensuring all pieces are thoroughly coated. Cover the bowl and refrigerate for at least 3 hours, or for best results, overnight (up to 12 hours).

Before grilling, remove the reserved 1/2 cup of marinade from the refrigerator. Pour it into a small saucepan. Bring to a gentle simmer over medium heat and cook for 5-7 minutes, stirring occasionally, until it slightly thickens into a glaze. This cooks the sauce, making it safe for basting. Set aside.

Preheat your grill to medium-high heat (approximately 375-400°F / 190-200°C). Lightly oil the grill grates to prevent sticking.

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

Place the marinated chicken thighs on the preheated grill. Grill for 6-8 minutes per side for boneless thighs, turning frequently (the "huli huli" motion) to ensure even cooking and charring.

During the last 10 minutes of grilling, begin to baste the chicken generously with the reduced Huli Huli sauce. Continue to turn and baste every few minutes until the chicken is beautifully caramelized and cooked through, reaching an internal temperature of 165°F (74°C) as measured with a meat thermometer.

Once cooked, remove the chicken from the grill and let it rest on a cutting board or platter for 5 minutes before serving. This allows the juices to redistribute, ensuring tender and flavorful chicken. Garnish with sesame seeds and chopped green onions if desired.