1. Make sauce: Combine ingredients in a medium saucepan. Cover and simmer 45 minutes to blend flavors, stirring occasionally. Add a little water if sauce gets too thick to pour. Let cool, then transfer to a plastic container and chill up to 1 week.

2. Prepare chicken: In a large bowl, coat chicken in oil; pack in a resealable plastic bag and chill up to 2 days, or freeze.

3. Build a charcoal or wood fire in a grill and let burn to medium (350° to 450°; you can hold your hand 5 in. above cooking grate only 5 to 7 seconds).

4. Place cast iron skillet over fire. Grill chicken in skillet until browned all over, about 15 minutes, turning occasionally. Turn again, generously brush tops with some of BBQ sauce, and cook a few minutes; repeat turning and brushing 2 more times, until chicken is well-browned and cooked through, 10 to 15 minutes total.

5. Remove from skillet and top with additional BBQ sauce. Serve immediately.