Preheat oven to 180C. Place potatoes on a tray lined with rock salt and bake until tender (45 minutes-1 hour). Working quickly while potatoes are hot, and holding them in a tea towel, scoop flesh from skin and pass through a mouli or potato ricer into a saucepan (see note). Add cavolo nero, spring onion, cream and butter and stir over low heat to warm through. Add parsley, season to taste, dress with olive oil and serve hot with crusty bread.