Bring a pot of water to a boil. Add the potatoes and cook for about 15 minutes, or until fork-tender.
Drain well and mash the potatoes until smooth.
Add the cornstarch, salt, black pepper, onion (or garlic) powder, and chopped parsley. Mix until a soft, dough-like texture forms.
Cut the mozzarella into small cubes.
Scoop a portion of the potato mixture, flatten it in your hand, place a cube of mozzarella in the center, and seal it completely. Roll into a smooth ball.
Heat oil to 350°F.
Fry the potato balls in batches until golden brown and crispy on the outside.
Remove and drain on paper towels. Serve warm for the best cheese pull.