Preheat oven to 200°C or 180°C fan. Butter a 20cm springform pan. Sprinkle base with one-third of breadcrumbs.
Cook potatoes in a large saucepan of salted water for 15 minutes, until tender. Drain and return to pan for 1 minute, until dry. Mash, adding milk and half of butter. Stir in parmesan, egg and egg yolk and season. Spread prepared pan with half of potato mixture. Cover with mozzarella and prosciutto and top with remaining potato mixture. Dot with remaining butter and sprinkle with remaining breadcrumbs. Bake for 30 minutes, until golden and warm. Stand cake for 10 minutes and serve with baby rocket.