In a mortar and pestle (or small bowl), combine the brown sugar, minced garlic, kosher salt, black pepper, smoked paprika, and cayenne if using. Mash and grind until you create a paste-like consistency. The garlic will break down and bind with the sugar, creating a mixture that adheres beautifully to the chicken.

Place your chicken pieces in a large resealable plastic bag or a shallow dish. Spoon the brown sugar rub paste over all the chicken pieces, then massage it into the meat, ensuring every surface is evenly coated. Gently loosen the skin and rub some of the mixture directly onto the meat for maximum flavor penetration.

If using a plastic bag, squeeze out as much air as possible before sealing. Refrigerate for at least 1 hour, though overnight marinating produces the best results. The longer marination time allows the salt to brine the chicken slightly while the sugar and spices infuse deep into the meat.

Preheat your gas grill to medium-high heat (around 375-400°F) or prepare a charcoal grill with coals pushed to one side for indirect grilling. Oil your grill grates well using tongs and a paper towel dipped in vegetable oil to prevent sticking.

Remove chicken from the refrigerator 15-20 minutes before grilling. Place chicken pieces on the cooler side of the grill (indirect heat), skin-side up. Close the lid and cook for 20-25 minutes without disturbing. This gentle cooking prevents flare-ups and allows the chicken to cook evenly.

After 20-25 minutes, flip the chicken pieces using tongs. Close the lid and continue cooking for another 15-20 minutes over indirect heat. The chicken should reach an internal temperature of about 155°F at this point when checked with an instant-read thermometer.

Once the chicken is nearly cooked through, brush a generous layer of BBQ sauce on both sides of each piece. Move the chicken to the direct heat side of the grill. Grill for 2-3 minutes per side, brushing with additional sauce each time you flip. Watch carefully as the sauce will caramelize quickly.

The chicken is done when it reaches an internal temperature of 165°F in the thickest part. Remove chicken from the grill and let it rest on a clean platter tented loosely with foil for 5-10 minutes. This resting period allows the juices to redistribute throughout the meat.