Pat the thighs dry, trim loose fat and open them flat. Season both sides with kosher salt and black pepper.
Combine the buttermilk and optional hot sauce. Coat the chicken and let it stand for 15 minutes.
Mix flour, cornstarch, paprika, garlic powder, onion powder, thyme, cayenne and fine salt. Rub in 2 tablespoons seasoned buttermilk to form pea-size flakes.
Let excess buttermilk drip away, then press each thigh firmly into the flour mixture until no wet spots show.
Preheat the air fryer to 380°F if recommended. Lightly mist the basket and spray the coated chicken until no dry flour remains.
Cook in one layer for 8 minutes. Turn, spray pale spots and cook for 6 to 8 minutes longer, until every thigh reaches 165°F in the thickest part.
Move the chicken to a clean rack and rest for 3 minutes before serving.