In a medium saucepan, whisk together the mustard, brown sugar and cider vinegar. Bring the mixture to a boil and then turn the heat to low and simmer.
Stir in the butter, water, fish sauce, chili powder, cumin, cayenne, garlic powder, salt and pepper. Simmer for 5 minutes until the mixture has thickened a bit. Turn off the heat and cool to room temperature. Set aside 1/2 cup of the sauce for serving. Use the rest for grilling.
GRILL THE CHICKEN:
Preheat your grill.
Place the chicken on the grill skin side down and cook for 4 to 5 minutes. Turn the chicken over and baste with the barbecue sauce. Cook for an additional 3 minutes.
Turn the chicken again and baste with the sauce. Cook for an additional 3 minutes. Repeat two more times.
Transfer the chicken to a platter and allow to rest for 5 minutes. Serve immediately with extra sauce on the side, if desired.