Peel and chop the starchy potatoes. Place them in a large pot of salted water and bring to a boil. Cook until fork-tender, about 15-20 minutes. Drain and cool slightly.

Heat a skillet over medium heat. Add a small amount of butter and finely chopped onions. Sauté until soft and translucent, about 5 minutes. Remove from heat and cool slightly.

In a large bowl, mash the cooled potatoes until smooth. Add eggs, pancetta (or mushrooms), sautéed onions, remaining butter, chives, salt, and pepper. If necessary, adjust consistency with milk.

Preheat the oven to 400°F (190°C). Line a baking sheet with parchment paper.

Shape the potato mixture into uniform cakes, about 1 inch thick, and place on the baking sheet. Sprinkle breadcrumbs over each cake.

Bake for 10-15 minutes until the tops are golden brown and crispy.

Cool for a few minutes before transferring to a platter. Enjoy warm, optionally with parmesan or dipping sauce.