In a large bowl, combine the brown sugar, paprika, cumin, garlic powder, onion powder, ground mustard, salt, chili powder, and black pepper.
Pat the chicken dry with paper towels.
Gently loosen the skin from the meat without removing it.
Rub the seasoning mixture all over the chicken, including underneath the skin for maximum flavor.
Let the chicken rest at room temperature for 20 minutes, or refrigerate for up to 24 hours for deeper flavor.
Preheat the grill to medium heat (about 350°F / 177°C).
Clean and lightly oil the grill grates.
Place the chicken skin-side down over direct heat.
Grill for 3–4 minutes until the skin develops grill marks and releases easily.
Flip the chicken and grill the other side for another 1–2 minutes.
Move the chicken to indirect heat or reduce the grill to low.
Brush the chicken with barbecue sauce.
Cover and grill for 5 minutes.
Flip, brush with more sauce, and continue cooking for another 10–15 minutes, brushing every 5 minutes until the Best-Ever Grilled BBQ Chicken reaches an internal temperature of 165°F (74°C).
Let the chicken rest for 5 minutes before serving with extra barbecue sauce.