Brine the chicken. In a bowl or zip-top bag, whisk the buttermilk, egg, 1 1/2 teaspoons salt, hot sauce and 1/2 teaspoon pepper. Add the tenderloins, coat, and refrigerate for 2 to 24 hours.
Mix the seasoned flour. In a shallow dish, whisk the flour, cornstarch, baking powder, 1 1/2 teaspoons salt, paprika, garlic powder, onion powder, 1/2 teaspoon pepper and cayenne. Drizzle in 2 tablespoons of the brine and rub it in with your fingers to form small clumps.
Bread the tenders. Lift one tender at a time from the brine, let it drip for a second, then press it firmly into the flour on both sides. Shake off the excess and set on a wire rack. Rest 10 minutes.
Spray with oil. Coat both sides generously with cooking oil spray until no dry flour shows.
Air fry. Preheat the air fryer to 390°F for 3 minutes. Arrange the tenders in a single layer without touching. Cook 7 minutes, flip, spray any pale spots, and cook 5 to 6 minutes more, until deep golden and 165°F inside.
Rest and serve. Let the tenders sit on the rack for 2 minutes, then serve with honey mustard, ranch, creamy buffalo or Carolina Gold BBQ.