Prepare the Marinade: In a large bowl, mix together pineapple juice, soy sauce, grated ginger, minced garlic, rice vinegar, brown sugar, hot sauce (if using), and ketchup. This mixture will serve as the marinade for the chicken.
Reduce the Marinade: Pour the marinade into a small saucepan and heat over medium-high. Allow it to boil, then reduce the heat to medium. Cook for 5-7 minutes, stirring occasionally, until the marinade reduces by about 25%. This step will intensify the flavors and slightly thicken the marinade.
Marinate the Chicken: Place the chicken thighs in a dish or a resealable plastic bag, and pour the reduced marinade over them. Make sure all the chicken is evenly coated. Cover the dish with plastic wrap or seal the bag and refrigerate for at least 2 hours, ideally overnight, to let the chicken absorb the flavors fully.
Grill the Chicken: Preheat your grill pan over medium-high heat. Once hot, place the chicken thighs on the grill and cook for about 10 minutes on each side, or until fully cooked and golden on the outside. While grilling, brush the chicken with the reserved marinade every 3 minutes for extra flavor.
Serve: Remove the chicken from the grill and let it rest for a few minutes before serving. Garnish with sesame seeds and serve with pineapple slices, white rice, or your favorite side dish. Enjoy this flavorful Hawaiian-inspired dish!