Cut string cheese into thirds to make 18 pieces.
In one bowl, mix flour, cornstarch, salt, and pepper.
In another bowl, whisk eggs and milk.
In a third bowl, combine panko breadcrumbs and parsley.
Coat each cheese piece in flour, then dip in egg, then roll in breadcrumbs.
Repeat dipping in egg and breadcrumbs, pressing gently to coat well.
Preheat air fryer to 350°F (175°C).
Freeze coated bites for 10–15 minutes while preheating.
Place bites in the air fryer basket, spray with cooking spray.
Cook Air Fryer Mozzarella Bites at 350°F for 8 minutes.
Shake basket halfway through cooking.
Serve immediately with marinara or ranch dip.