Prepare the Potatoes: Boil the potatoes in their skins until tender. Peel and mash them, leaving some texture.
Cook the Guanciale: Dice the guanciale and brown it in olive oil. Set aside.
Mix Ingredients: Combine the mashed potatoes with the cooked guanciale, melted fat, Parmesan cheese, egg yolk, salt, and pepper.
Assemble the Tortino: Spread half the mixture in a greased baking dish. Add a layer of mozzarella, then top with the remaining potato mixture.
Add Topping: Sprinkle breadcrumbs and dot with butter.
Bake: Bake at 200°C (400°F) for 35-45 minutes until golden brown.
Serve: Let cool slightly before slicing. Serve warm as a main or side dish.