Marinate the meat by placing the chicken thighs and buttermilk in a large zip-lock bag, sealing it, and squishing it to coat the chicken evenly. Place in the refrigerator to marinate for at least 30 minutes and up to several hours.
Remove the chicken from the fridge 30 minutes before baking.
Heat a baking sheet by placing it in the oven and preheating it to 425 degrees, using the convection setting if you have it.
Make the coating by combining the flour, panko crumbs, white pepper, celery salt, Italian seasoning, salt, garlic powder, black pepper, dried mustard, paprika, ginger, baking powder, turmeric, and cayenne in a shallow bowl. Mix until well-combined.
Dredge the chicken thighs by removing them, one at a time, from the buttermilk, shaking off excess liquid, and dabbing both sides with a few paper towels, then place them skin-side down into the prepared panko crumb mixture. Coat both sides of the chicken well, then place on a parchment-lined plate. Repeat with the remaining chicken pieces. Spray the top of the thighs with avocado or olive oil cooking spray.
Bake the chicken thighs by removing the HOT baking sheet from the oven once preheated. Carefully pour the melted butter evenly over the HOT baking sheet. Place the chicken thighs skin-side down, separated from each other so they will "fry" instead of steam–they all must have space! Bake for 15 minutes.
Carefully flip them over, then bake for another 15-17 minutes, or until cooked to an internal temperature of 165 degrees using a meat thermometer.
To serve, remove from the oven, sprinkle with a bit of sea salt (if desired), and rest for 5 minutes before serving. Enjoy.