Step 1: Prepare the Marinade: In a large bowl, whisk together the pineapple juice, soy sauce, brown sugar, rice vinegar, grated ginger, minced garlic, sesame oil, smoked paprika, black pepper, and red pepper flakes (if using). Ensure the brown sugar is fully dissolved.
Step 2: Marinate the Chicken: Place the chicken pieces (or spatchcocked whole chicken) in a large resealable bag or a non-reactive container (glass or ceramic). Pour the marinade over the chicken, ensuring all pieces are fully coated. Seal the bag or cover the container and refrigerate for at least 4 hours, or preferably overnight, turning the chicken occasionally to ensure even marination.
Step 3: Prepare the Grill: Preheat your grill to medium heat (about 350-400°F or 175-200°C). Make sure the grill grates are clean and lightly oiled to prevent sticking.
Step 4: Remove Chicken from Marinade: Take the chicken out of the refrigerator about 30 minutes before grilling to allow it to come closer to room temperature. Remove the chicken from the marinade and reserve the marinade in a saucepan.
Step 5: Simmer the Reserved Marinade: Pour the reserved marinade into a saucepan and bring it to a boil over medium heat. Reduce the heat and simmer for 10-15 minutes, or until the marinade has thickened slightly. This step is crucial to kill any bacteria from the raw chicken and create a delicious glaze.