A gloriously rich and light mash from ” Nigellissima” by Nigella Lawson
Put the potato in a steamer above a man of boiling water filled about 8 cm deep and steam for about 20 minutes until soft.
Remove the steamer, pour out all the remaining water from the pan and put the steamer and cooked potatoes back on top of the pan with the lid off to stand for 10 minutes to dry out.
Remove the top part and warm the milk, without allowing to boil, in the bottom part of the steamer. Stir in the mascarpone and the salt.
Tip in the potatoes and mash, then beat with a wooden spoon , whipping in some air as you do.
When smooth beat in the Parmesan and white truffle paste or truffle butter, tasting as you go. If using truffle oil just a drop or two will probably be enough.