Step 1: Prepare the Marinade: In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, ketchup, rice vinegar, grated ginger, minced garlic, sesame oil, and black pepper. Add red pepper flakes if desired.
Step 2: Marinate the Chicken: Place the chicken pieces in a large resealable bag or a shallow dish. Pour the marinade over the chicken, ensuring all pieces are well-coated. Seal the bag or cover the dish and refrigerate for at least 2 hours, or preferably 4-8 hours for maximum flavor. You can marinate overnight if preferred.
Step 3: Prepare to Cook: Remove the chicken from the refrigerator about 30 minutes before cooking to bring it closer to room temperature. Reserve about 1/2 cup of the marinade in a small saucepan for basting; discard the rest of the used marinade.
Step 4: Make the Basting Sauce: Bring the reserved marinade to a simmer over medium heat on the stovetop. Cook for 5-7 minutes, stirring occasionally, until it thickens slightly into a glaze. Set aside.
Step 5: Grill the Chicken (Recommended): Preheat your grill to medium-high heat (around 375-400°F / 190-200°C). Lightly oil the grill grates. Place the chicken on the grill, discarding any remaining unused marinade. Grill for 6-8 minutes per side, basting generously with the thickened glaze during the last 10 minutes of cooking, until the chicken is cooked through and reaches an internal temperature of 165°F (74°C). The chicken should be beautifully caramelized and slightly charred.
Step 6: Bake the Chicken (Alternative Method): Preheat your oven to 400°F (200°C). Line a baking sheet with foil and place an oven-safe rack on top. Place the chicken on the rack. Bake for 15 minutes, then flip the chicken, baste with the thickened glaze, and bake for another 15-20 minutes, or until cooked through and caramelized. Baste every 5-7 minutes during the last 15 minutes of baking.
Step 7: Rest and Serve: Once cooked, remove the chicken from the grill or oven and let it rest for 5 minutes before serving. Garnish with chopped green onions and serve with white rice and fresh pineapple slices for an authentic Hawaiian experience.