1-Make the sauce: Melt butter in a saucepan over medium heat, cook onion and garlic until softened (about 3 minutes). Add ketchup, mustard, apple cider vinegar, Worcestershire sauce, paprika, chili powder, sriracha, sambal oelek, and brown sugar. Stir, bring to a simmer, and cook 15 minutes until thickened. Remove from heat and set aside.

2-Prepare the chicken: Pat chicken thighs dry with paper towels. Season both sides with salt and pepper, then drizzle with olive oil and rub to coat evenly.

3-Preheat the grill: Heat grill to medium-low (350-375°F). Oil the grates well. For charcoal, use a two-zone setup with coals banked on one side.

4-Grill the chicken: Place thighs on grill over direct heat. Cook 3-4 minutes per side, then move to indirect heat if using. Continue grilling, turning every 3-4 minutes, until internal temperature reaches 165-170°F (15-20 minutes for boneless, 25-30 for bone-in).

5-Baste with sauce: During the last 5-10 minutes of cooking, brush the spicy BBQ sauce onto both sides of the chicken. Turn once more to let the sauce set, being careful not to burn.

6-Rest and serve: Transfer grilled chicken to a plate and let rest for 5 minutes. Serve with extra sauce on the side.