Marinate the thighs with the buttermilk and 1/2 teaspoon salt for 1 to 6 hours in the refrigerator.
Preheat the air fryer to 380°F for 4 minutes and combine the flour, cornstarch, paprika, garlic powder, pepper and remaining salt.
Shake excess buttermilk from the thighs and press them into a single thin coat of flour mixture.
Arrange half the thighs in the basket without overlap and spray every visible dry flour patch with oil.
Air-fry for 9 minutes, turn, spray any new dry patches and cook for 7 to 9 minutes more.
Remove the thighs when the coating is crisp and each thickest piece reaches at least 165°F.
Rest for 5 minutes and repeat with the remaining chicken after reheating the empty fryer for 2 minutes.