Trim any excess fat from the chicken thighs and pat them dry with paper towels.
Rub the chicken thighs with olive oil to help the spices adhere.
Season the thighs with salt, black pepper, and garlic powder. Let them sit at room temperature while you prepare the sauce.
In a medium saucepan, combine the hot sauce, ketchup, apple cider vinegar, honey, brown sugar, Worcestershire sauce, minced garlic, smoked paprika, and cayenne pepper (if using).
Cook over medium heat, stirring occasionally, until the mixture starts to bubble. Reduce the heat to low and simmer for about 10-15 minutes until the sauce thickens slightly.
Remove from heat and stir in the melted butter until the sauce is smooth and well-blended. Let it cool slightly.
Place the chicken thighs in a large resealable plastic bag or a shallow dish.
Pour about half of the Buffalo BBQ sauce over the chicken thighs, ensuring they are well-coated. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to overnight for best results.
Preheat your grill to medium-high heat (about 375-400°F).
Oil the grill grates to prevent sticking.
Remove the chicken thighs from the marinade, letting any excess drip off. Discard the marinade.
Place the chicken thighs on the grill, skin-side down. Grill for about 5-7 minutes, or until the skin is crispy and has nice grill marks.
Flip the chicken thighs and move to indirect heat. Close the lid and continue cooking for another 20-25 minutes, or until the internal temperature reaches 165°F.
During the last 10 minutes of grilling, brush the chicken thighs with the remaining Buffalo BBQ sauce.
Once the chicken thighs are done, remove them from the grill and let them rest for about 5 minutes.