In a large bowl, combine the ground beef, italian seasoned breadcrumbs, parmesan cheese, milk, egg, garlic, parsley, italian seasoning, salt, and black pepper. Mix gently with your hands until everything is just combined. Be careful not to overwork the meat or the meatballs will become dense.
Roll the mixture into approximately 16 meatballs, each about 1 1/2 inches in diameter. Try to keep them uniform in size so they cook at the same rate.
Preheat your air fryer to 375 degrees F. Place the meatballs in a single layer in the basket, leaving a little space between each one. Air fry for 10 to 12 minutes, shaking the basket gently halfway through, until the meatballs are browned on the outside and cooked through to an internal temperature of 160 degrees F.
While the meatballs cook, lightly spread the softened butter on the cut sides of each hoagie roll. Once the meatballs are done, remove them and set aside. Place the rolls cut-side up in the air fryer basket and toast at 375 degrees F for about 2 minutes until lightly golden and crisp.
Remove the toasted rolls and spoon a generous layer of warm marinara sauce along the bottom of each roll. Nestle 4 meatballs into each sub, then spoon a bit more marinara over the top of the meatballs.
Lay a slice and a half of provolone cheese over the meatballs on each sub. Return the open-faced subs to the air fryer at 375 degrees F for about 1 to 2 minutes, just until the cheese is melted and bubbly.
Remove the subs from the air fryer, garnish with fresh basil leaves, close the rolls, and serve immediately while hot and gooey.