Cut the chicken into 10 pieces. Place in a large bowl and season with 1 teaspoon of the kosher salt and 1/2 teaspoon of the black pepper. Add 2 cups buttermilk and marinate for at least 1 hour in the refrigerator. Meanwhile, whisk the remaining 1 tablespoon kosher salt, remaining 1/2 teaspoon black pepper, 2 cups all-purpose flour, 1 tablespoon garlic powder, 2 tablespoons paprika, 1 teaspoon cayenne pepper, 1 tablespoon onion powder, and 1 tablespoon ground mustard together in a large bowl.
Preheat an Instant Vortex Plus 7-in-1 Air Fryer Oven to 390ºF. 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 165ºF in the thickest piece, 18 to 20 minutes total. Repeat with the remaining chicken.