01 - Combine chicken pieces with fish sauce, soy sauce, brown sugar, black pepper, garlic, and shallot in a large bowl. Mix thoroughly and marinate for at least 15 minutes, up to 1 hour for deeper flavor penetration.
02 - Heat oil in a large skillet or clay pot over medium heat. Add brown sugar and cook while stirring constantly until melted and deep amber in color, approximately 2-3 minutes.
03 - Add marinated chicken to the caramel, reserving excess marinade in the bowl. Sear for 3-4 minutes, turning pieces to coat evenly in the caramelized sugar.
04 - Pour in reserved marinade, water, fish sauce, soy sauce, and sliced chilies if using. Stir thoroughly to combine all ingredients.
05 - Reduce heat to low, cover, and simmer for 25-30 minutes. Stir occasionally to prevent sticking and ensure even cooking. Chicken is done when meat pulls easily from the bone and sauce thickens to a glossy consistency.
06 - Remove the lid during the final 5 minutes of cooking to allow the sauce to reduce and concentrate flavors further.
07 - Transfer to a serving dish and top with sliced scallions and fresh cilantro. Serve immediately alongside steamed jasmine rice.