In a large bowl, whisk together the fish sauce, brown sugar, soy sauce, lime juice, garlic, ginger, black pepper, and red pepper flakes until the sugar is dissolved.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, or up to 4 hours for more flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade and let any excess drip off. Discard the marinade.
Grill the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F and the chicken is nicely charred.
Remove the chicken from the grill and let it rest for 5 minutes before slicing.
Serve the grilled chicken garnished with fresh cilantro and lime wedges on the side.