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Peel the potatoes and cut into even size dice so they all cook at the same time – about 12-15 minutes in boiling salted water. Drain well then return to the pan and cook for 1-2 minutes over a dry heat. Mash the potatoes very thoroughly or press them through a potato ricer or mouli (vegetable mill) back into the pan – they should be completely lump free. Don't be tempted to use a food processor though, as it will make the potatoes gluey.
Gradually beat 100g/4oz of the butter into the potato until it starts to look shiny. Heat the milk in a small pan and cream until on the point of boiling, then slowly mix into the potato purée with some salt and freshly ground pepper. The potato should become a soft velvety purée. You may not need all the creamy milk. Add the truffle oil and seasoning to taste. Keep warm until required or, if making ahead, cool and chill. When reheating, add a little extra creamy milk to soften the texture, and the last of the butter if you like.