1. In a large bowl, combine ketchup, soy sauce, calamansi juice (or lemon/lime juice), brown sugar, minced garlic, grated ginger, and black pepper. 2. Whisk all ingredients together until the brown sugar is fully dissolved and the marinade is well combined. 3. Add your preferred chicken pieces (about 2-3 lbs) to the bowl, ensuring they are fully coated in the marinade. For larger batches, divide chicken into separate bags or bowls. 4. Cover the bowl or seal the bag and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to penetrate the chicken. 5. When ready to cook, preheat your grill, air fryer, or oven. Remove chicken from the marinade, reserving some marinade for basting if desired (ensure reserved marinade is boiled for 5 minutes before using for basting). 6. Grill chicken over medium heat for 20-25 minutes, turning occasionally, until cooked through and nicely caramelized. Alternatively, air fry at 375°F (190°C) for 18-22 minutes, flipping halfway, or bake at 400°F (200°C) for 25-35 minutes. 7. Serve hot with steamed rice and enjoy your authentic Filipino BBQ Chicken!