Place chicken in a large bowl and season with 1 teaspoon of kosher salt and 1/2 teaspoon of black pepper. Add 2 cups buttermilk and marinate for at least 1 hour in the refrigerator. Overnight is better.
Meanwhile, whisk 1 tablespoon kosher salt, remaining 1/2 teaspoon black pepper, all-purpose flour, garlic powder, paprika, cayenne pepper, onion powder, and ground mustard together in a large bowl.
Preheat an air fryer to 390F. Coat the air fryer racks with cooking spray.
Remove the chicken from the buttermilk, allowing any excess to drip off. Dredge in the flour mixture, shaking any excess off. Place a single layer of chicken in the basket, with space in between the pieces.
Air fry, flipping the chicken hallway through, until crispy and an instant-read thermometer registers 165F in the thickest piece, 15 to 20 minutes total. Repeat with the remaining chicken.