Use a cookie scoop to place bite-sized balls of leftover mac and cheese onto a plate. Freeze for 20 minutes.
In a shallow dish, whisk egg and milk together. In another shallow dish, combine flour and seasonings. Place breadcrumbs into another shallow dish.
First coat mac and cheese bites in egg mixture, roll in flour mixture, back into eggs, and fully coat in breadcrumbs. Set back onto plates. Freeze for another 20 minutes.
Preheat air-fryer to 400 F. Air-fry mac and cheese bites in batches for 8-10 minutes.