Cook potatoes in boiling salted water until tender. Drain, cool slightly, peel and cut into 1.5cm pieces, drizzle with 2 tbsp olive oil and season to taste.
Heat remaining olive oil in a large frying pan, add garlic and sauté for 2 minutes, add cavolo nero and cook until wilted.
Cut fresh pasta into 42cm lengths, join 3 side by side by brushing long edges with egg and overlapping slightly, then press to join edges to form a single piece (approximately 38cm x 42cm).
Place pasta sheet on a clean tea towel, then lay salami in a single layer leaving a 2cm border, top with potato, then cavolo nero. Starting at long edge, roll tea towel firmly to enclose pasta and filling. Secure both ends of tea towel with string, then tie at 5cm intervals.
Bring a fish kettle or large, wide saucepan of salted water to the boil, place rotolo in pan and simmer gently for 20 minutes. Using tongs, remove rotolo from pan and stand on a chopping board for 5 minutes. Unwrap, trim ends, and, using a sharp knife, cut rotolo into 5cm-thick slices.
Meanwhile, cook butter and rosemary in a frying pan over medium heat until dark golden, then season to taste. Place rotolo slices on plates, drizzle with butter, scatter with parmesan and serve immediately.