Preheat oven to 450 F.

Fill a pasta pot with water and add the whole potatoes. Bring the water to a boil and cook until tender, about 45 minutes. Drain and peel the potatoes and pass them through a food mill. While the potatoes are still warm, place them in a mixing bowl and combine the eggs, ricotta, cheeses, soppressata, milk and parsley. Stir just enough to evenly mix.

Butter a 12-inch springform pan with 2 tablespoons of butter and dust with breadcrumbs. Place half of the potato mixture into the pan and gently smooth it to the edges. Sprinkle the mozzarella over the potato mixture, to within 1/2 inch of the outer edge, but not over. Smear the remaining potato mixture over carefully, and smooth with a wet spatula. Sprinkle with remaining bread crumbs and dot with remaining butter.

Place the gatto in the oven and cook 25 to 30 minutes, until light golden brown on top. Remove from oven, let stand 15 minutes, unmold and serve in slices.