Slice each piece of cheese in half crosswise so that you have 24 shorter pieces. Arrange the cheese in a single layer on a parchment-lined baking sheet, cutting board, or plate, then freeze until completely solid, about 1 hour.
Once the cheese is frozen, place the flour in a pie dish or similar shallow bowl. In a separate shallow bowl, whisk the egg and milk.
In a third shallow bowl, combine the Italian breadcrumbs, panko, Italian seasoning, and garlic powder. Arrange the plates in an assembly line: flour, eggs, breadcrumbs. Line a large, rimmed baking sheet with parchment paper.
Working to 2 to 3 cheese sticks at a time, dredge the frozen cheese sticks in the flour, shaking off excess. Next, dip into the egg, then place in the bowl with the crumbs and lightly pat to coat (don’t be tempted to tackle too many cheese sticks at a time; the egg will freeze right onto the cheese and no longer be sticky enough for the crumbs to adhere).
Place the breaded cheese on the prepared baking sheet, then repeat the process until all of the cheese sticks are coated. Place cheese back into the freezer for at least 1 hour (this is a MUST or the cheese will melt before the crumbs are golden).
When ready to bake, preheat your air fryer to 390 degrees F. Generously coat the basket with nonstick spray. Arrange the frozen cheese sticks in a single layer on top, ensuring none are touching (you will need to cook them in batches). Coat the tops of the sticks with more spray.
Air fry mozzarella sticks for 6 to 8 minutes, until the cheese is heated through and the crumbs are golden. Remove to a plate and let stand for 3 minutes (they are HOT; you’ve been warned). Repeat with remaining batches. Serve warm with marinara sauce (heat the sauce first for the best experience) or ranch for dipping.
*You can also prepare this recipe with a block of mozzarella cheese rather than string cheese. Simply cut the block into 4-inch by 1/2-inch sticks.
TO MAKE AHEAD AND FREEZE: Bread the sticks as directed, then freeze until hardened. Transfer to a ziptop bag and freeze them for up to 3 months. Bake directly from frozen in the air fryer, adding to the baking time as needed.
TO STORE: Mozzarella sticks are best enjoyed immediately after making. You can try saving the leftovers in an airtight container in the refrigerator for up to 2 days.
TO REHEAT: Gently rewarm the mozzarella sticks on a baking sheet in a 350 degrees F oven until warmed through.
TO FREEZE: I recommend freezing the cheese sticks before baking if possible. If you'd like to freeze them after baking, place the mozzarella sticks on a parchment-lined baking sheet and freeze until solid. Transfer the frozen sticks to an airtight freezer-safe storage container for up to 3 months. Reheat from frozen on a baking sheet in the oven at 350 degrees F.
Nutritional information has been calculated with half the amount of flour and bread crumb mixture, as not all of it sticks to the cheese.