In a large bowl, combine soy sauce, fish sauce, brown sugar, vegetable oil, garlic, ginger, ground coriander, ground cumin, black pepper, cayenne pepper (if using), and lime juice. Whisk until the sugar is dissolved and the marinade is well mixed.

Add the chicken thighs to the marinade, ensuring they are fully coated. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or preferably overnight for maximum flavor.

Preheat your grill to medium-high heat. If using a charcoal grill, prepare the coals until they are covered with white ash.

Remove the chicken from the marinade and let any excess drip off. Discard the marinade.

Place the chicken skin-side down on the grill. Cook for about 6-8 minutes, then flip and cook for another 6-8 minutes, or until the internal temperature reaches 165°F and the skin is crispy.

Remove the chicken from the grill and let it rest for 5 minutes before serving. Garnish with fresh cilantro.