Boil the potatoes: Cook potatoes in salted water until very tender. Drain well and mash until smooth.
Make the filling: While warm, mix potatoes with butter, Parmesan, egg yolk, salt, pepper, and parsley. Let cool slightly until firm enough to shape.
Wrap the cheese: Take a portion of potato mixture and flatten it. Place a mozzarella stick in the center and completely surround it with potato. Seal all edges carefully.
First freeze: Place the potato-wrapped mozzarella sticks on a parchment-lined tray and freeze for at least 30 minutes, until firm.
Prepare breading: Mix panko, Parmesan, garlic powder, salt, and pepper in one bowl. Place flour in a second bowl and beaten eggs in a third.
Bread: Coat each frozen stick in flour, then beaten egg, then the panko-Parmesan mixture. Press the breadcrumbs on firmly.
Double coat: For an extra-crispy shell, dip again into egg and then panko.
Second freeze: Freeze the breaded sticks for another 20–30 minutes.
Fry: Heat oil and carefully fry the sticks in batches until deeply golden brown and crispy.
Drain: Transfer to a wire rack or paper towel-lined plate and let excess oil drain.
Serve: Serve hot with warm marinara sauce.
