Cut each string cheese into thirds to make 18 equal bites.
In a shallow bowl, whisk flour, cornstarch, salt, and pepper.
In a second bowl, beat eggs and milk until smooth.
In a third bowl, mix panko breadcrumbs with dried parsley.
Roll each cheese piece in flour, then dip in egg wash.
Coat with breadcrumbs, pressing gently to adhere.
Repeat the egg wash and breadcrumb coating for a double layer.
Place coated bites on a plate and freeze for at least 5 minutes.
Preheat air fryer to 350°F (175°C).
Arrange frozen bites in a single layer in the air fryer basket.
Spray bites generously with nonstick cooking spray on all sides.
Air fry at 350°F (175°C) for 8 minutes, shaking gently halfway through.
Serve hot with warm marinara sauce for dipping.