To make the marinade, whisk together the brown sugar, ketchup, soy sauce, rice vinegar, ginger, and garlic together in a sealable plastic bag or large dish. Reserve ½ cup of the marinade for cooking.
Place the chicken in the marinade and let it marinate overnight for the best flavor or at least 2 hours.
Oil the racks of the grill. Heat the grill to medium. Place the chicken on the grill, cooking for 20-25 minutes. Every 5 minutes, flip the chicken and baste with the reserved marinade. The chicken is finished once the internal temperature of the thickest part of the thigh or breast reaches 165°F.
Finish with a sprinkling of chopped cilantro and green onions. Serve with grilled pineapple if desired.
