In a small saucepan over medium heat, melt 2 tablespoons of butter until bubbly. Add 1 finely chopped yellow onion and sauté for 5-7 minutes until translucent. Stir in 3 cloves of minced garlic and sauté until fragrant, about 1 minute. Mix in ketchup, apple cider vinegar, water, paprika, Sriracha, hot chili paste, chili powder, mustard, and Worcestershire sauce. Simmer for 15 minutes until thickened.

While the barbecue sauce simmers, take 4 boneless skinless chicken thighs and place in a bowl. Drizzle with 2 tablespoons of olive oil, then season generously with salt and black pepper. Rub to coat evenly and set aside to marinate.

Preheat your grill to medium-low heat, aiming for around 300°F. Lightly oil the grill grates.

Place the seasoned chicken thighs smooth side down on the grill. Cook for about 6-8 minutes, then flip. Continue grilling for another 6-8 minutes, turning every 3-4 minutes, until juices run clear and internal temperature reaches 165-170°F.

During the last few flips of grilling, brush the chicken thighs with the homemade spicy BBQ sauce. Grill for another 2-3 minutes on each side.