Wash the potatoes thoroughly. Peel and cut them into small pieces.
Place the potatoes in a pot with enough water. Boil for about 15–20 minutes or until fork tender.
Drain the potatoes well. Mash until completely smooth.
Transfer the mashed potatoes into a bowl. Add the all-purpose flour, glutinous rice flour, powdered milk (if using), sugar, salt, and black pepper. Mix well, then knead until a soft dough forms.
Divide the dough into portions weighing about 50 grams each.
Flatten one portion. Place a cube of mozzarella cheese in the center. Seal completely and shape into a ball.
Flatten the ball slightly into a thick round patty.
Dip each potato patty into the beaten egg, then coat evenly with breadcrumbs.
Heat enough cooking oil over medium heat. Fry each piece until golden brown and crispy on both sides.
Drain on paper towels.
Top with mayonnaise, ketchup, or hot sauce if desired. Serve immediately while the cheese is still melted.