Preheat oven to 425°F.
Mix the salt, pepper, and garlic powder (if using) in a small bowl.
To make the most out of the seasoning, lift the skin and sprinkle the seasoning directly on the meat below the skin. This way, you will flavor the meat and not the outside of the skin. The rest of the spice mixture goes on the skinless part of the thigh.
Put the chicken thighs skin down into the cold pan and turn on the burner to medium.
Once the skin starts to crackle and pop, cook for about 6 minutes without moving the thighs.
After 6 minutes, flip the thighs to skin side up.
After you flip the thighs skin side up, place the cast iron pan directly on the center rack of the preheated oven and bake for about 30 minutes.
Remove the pan and use a digital meat thermometer to make sure the internal temperature reaches at least 160°F at the thickest part (without touching the bone).