In a small bowl combine hoisin sauce, ketchup, rice vinegar, honey, soy sauce, sriracha and pepper. Reserve half to serve with chicken.
Place chicken thighs in a shallow dish. Baste thighs liberally with BBQ sauce. Cover dish and refrigerate for 1 hour, or marinate overnight.
Preheat one side of the BBQ grill on the low heat setting.
Place chicken thighs, skin side up on the opposite side of the grill away from direct heat. Cooking them low and slow will render out the fat, sealing in the juices, and crisp up the skin as it cooks.
Close lid and check chicken after the first 30 minutes. If chicken is showing signs of burning, reduce heat further and move farther away from flame.
After another 30 minutes, flip chicken over, skin side down, and baste chicken with Hoisin BBQ sauce.
After another 30 minutes, flip chicken over and baste again. Cover and continue to cook over low heat.
Continue cooking, flipping chicken every 30 minutes and basting the chicken until meat is cooked through and tender. It will take a good 2 to 2½ hours to cook the chicken all the way through using low heat.
Flip chicken, skin side down, and move thighs to the other side of the BBQ directly over the flame. Crank up the heat to medium-high and sear the chicken. This will get the skin really nice and crispy. Keep a close eye on them, they will burn if left over the flame for more than a minute or two.
Turn off heat and finish off thighs with a final baste of BBQ sauce.
Serve with the reserved BBQ sauce and a big pile of napkins.