Prepare Marinade: In a medium bowl, whisk together soy sauce, banana ketchup, brown sugar, minced garlic, chopped onion, vinegar, black pepper, salt, and cooking oil until you have a glossy, smooth mixture, ensuring the sugar is fully dissolved.

Marinate Chicken: Add the chicken pieces into the marinade, ensuring each piece is thoroughly coated. Massage the marinade into the chicken, cover, and refrigerate for at least 3 hours, preferably overnight.

Preheat Grill: Preheat your grill to medium-high heat, allowing charcoal to ash over for about 20-30 minutes if using.

Skewer Chicken: Remove chicken from marinade, reserving it for basting. Thread chicken onto soaked bamboo skewers, leaving space between pieces for cooking.

Grill Chicken: Place skewers on the grill, cooking for approximately 10-15 minutes on each side, basting with reserved marinade until the internal temperature reaches 165°F (75°C).

Rest and Serve: Let skewers rest for a few minutes, then serve hot, optionally on banana leaves or with steamed rice.