Prep the chicken. Pat the chicken breasts very dry with paper towels. If one end is much thicker than the other, gently pound to an even ¾-inch thickness.
Make the seasoned oil. In a small bowl, stir together the olive oil, chopped rosemary, garlic powder, salt, pepper, and smoked paprika (if using).
Season. Brush or rub the seasoned oil all over both sides of the chicken, making sure every spot is coated.
Preheat the air fryer to 380°F (190°C) for 3 minutes.
Air fry. Place chicken in the basket in a single layer with space between them. Cook at 380°F for 9 minutes.
Flip and finish. Flip with tongs and cook another 7-9 minutes, until an instant-read thermometer reads 155°F (68°C) in the thickest part.
Rest. Transfer to a plate, tent loosely with foil, and rest for 5 minutes. Temperature rises to a safe 165°F (74°C).
Slice and serve. Slice across the grain into ½-inch pieces. Sprinkle with flaky salt and extra rosemary if you like.