In a mortar and pestle, mash together the brown sugar, minced garlic, salt, and black pepper until it forms a paste.

Transfer the sugar-garlic paste to a large resealable plastic bag. Add the chicken pieces and massage the bag to coat all pieces evenly with the paste. Marinate for at least 1 hour or overnight.

Combine all BBQ sauce ingredients in a small saucepan. Simmer over low heat for about 10 minutes, stirring occasionally, until slightly thickened. Set aside to cool.

Preheat your grill to medium heat (around 350-375°F). Create direct and indirect heat zones.

Remove chicken from the marinade and pat dry with paper towels. Brush with olive oil and place skin-side down on the direct heat side of the grill for 5-6 minutes.

Move the chicken to the indirect heat zone, skin side up. Close the grill lid and continue cooking for about 20-25 minutes.

Start basting the chicken with your homemade BBQ sauce. Apply a generous coat, close the lid, and cook for 5 minutes. Repeat this process 2-3 more times until the chicken reaches an internal temperature of 165°F for breasts and 175°F for thighs and drumsticks.

Transfer the grilled chicken BBQ to a clean platter and let it rest for 5-10 minutes before serving.