1) Place the potatoes and 1 tbsp of salt in a 4-litre saucepan and add cold water to cover. Bring to a boil, lower the heat and simmer covered for 25 to 35 minutes, until completely tender.
2) Drain the potatoes and put them into the bowl of an electric mixer fitted with a paddle attachment and mix for a few seconds on low speed, to break them up.
3) In a small saucepan, heat the single cream and butter. Slowly add the hot cream and butter to the potatoes, mixing on the lowest speed (the last quarter of the cream should be folded in by hand).
4) Fold in the sour cream, Parmesan cheese, the remaining salt, and pepper; taste for seasoning and serve immediately. If the potatoes are too thick, add more hot cream and butter.