Wash the cavolo nero, strip the leaves from the ribs and then chop the leaves into thick ribbons.
Warm the olive oil, butter and garlic in heavy based frying pan, add the peperoncino and the cavolo nero and some salt. Cook over low heat, uncovered, for about 10 minutes by which point the cavolo nero should be wilted
Add a cup of water, partially cover the pot and let it cook for another half hour and the cavolo nero is very soft and tender.
In the meantime, put the ricotta in a serving dish and bring a big pan of well salted water to a fast boil for the pasta. When the cavolo nero is cooked, add it to the ricotta and mix well.
Put the pasta into the boiling water and cook until al dente. Spoon some of the starchy pasta water from the boiling pan into the cavolo nero/ricotta mixture to thin it into a creamy, loose sauce (I used about 4 tbsp of pasta water)
Drain the pasta and toss with the cavolo nero and ricotta.Grate a very generous amount of parmesan over the pasta and toss well before serving and eating immediately.