Peel and slice the potatoes into quarters and place in a large stock pot. Cover the potatoes with at least 2 inches of cold water. Add salt and bring the potatoes to a simmer. Do not bring to a full boil. Simmer the potatoes until fork tender, about 40 minutes.
Strain the potatoes through a colander, reserving ¼ cup of the salted cooking water. Allow the potatoes to cool in the colander for 2 minutes.
Process the potatoes through a food mill directly back into the stock pot. Add the melted butter. Mix in the crème fraîche and sour cream. Add the heavy cream and/or salted water until the potatoes are thick and creamy, but do not over-stir as they will become gluey Adjust seasoning as needed.
Shave truffle over the potatoes and mix in gently. Add truffle oil if desired. Serve immediately.
