1) Stir together hoisin, ketchup, rice wine vinegar, honey, soy sauce, sriracha, and black pepper until smooth. Divide the BBQ sauce in half. Use half while the chicken cooks and half as a sauce for serving later.
2) Brush chicken thighs lightly with the hoisin BBQ. Place them on a grill over low heat (indirect heat is best) skin-side up. Cook for about 30-40 minutes.
3) Check chicken thighs for scorching or if they are getting too black. If necessary, flip the chicken thighs so they don’t get too black on one side. Brush the thighs with more BBQ sauce 2-3 times as they cook. They will need to grill for at least two hours to be cooked through, but you could let them go as long as four hours if you have a nice low heat.
4) Optional ways to control heat: stack the chicken thighs in a big pile in the center of the grill. Rotate them around top-to-bottom every 15-20 minutes, but this will keep them warm without drying them out. You can also crank the heat up to medium-high near the end and cook the chicken thighs over high heat, skin side down for 2-3 minutes to crisp up the skin even more.
The chicken thighs should reach 165-170 degrees F. in the thickest part when they are done. Serve thighs with extra sauce on the side and garnish with scallions if you want.