In a large bowl, stir the breadcrumbs and milk together. Let the mixture sit for five minutes so the breadcrumbs soften.
Add the ground beef, egg, Parmesan cheese, minced garlic, chopped parsley, Italian seasoning, kosher salt, and black pepper to the bowl. Mix gently with your hands until the ingredients are just combined.
Preheat your air fryer to 380°F (190°C).
Scoop the meat mixture and roll it into one and a half inch balls. You should get about twenty meatballs.
Lightly coat the air fryer basket with cooking spray. Place the meatballs in a single layer with a little space between each one. Cook in batches if necessary.
Air fry at 380°F (190°C) for 10 minutes. Shake the basket gently halfway through the cooking time. The meatballs are done when they are browned on the outside and reach an internal temperature of 160°F (71°C) on a meat thermometer.
While the meatballs cook, pour the marinara sauce into a saucepan and warm it over medium heat.
Transfer the cooked meatballs into the warm sauce and simmer for 3 to 5 minutes.
Top the meatballs with shredded mozzarella cheese and extra parsley before serving.