Preheat oven to 400 degrees F. To make the sauce, whisk together chili sauce, soy sauce, garlic, fish sauce (or anchovy paste), ginger, lime juice, and Sriracha in a small bowl; set aside. Melt butter in a large oven-proof skillet over medium high heat. Add chicken, skin-side down, and sear both sides until golden brown, about 2-3 minutes per side. Stir in chili sauce mixture. Place into oven and roast until completely cooked through, reaching an internal temperature of 165 degrees F, about 30-40 minutes depending on the size of the thighs. Baste with sauce then broil for 2-3 minutes, or until caramelized and slightly charred. Serve immediately, garnished with peanuts and cilantro, if desired.