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In a large pot, cover the potatoes with cold water and bring to a boil. Salt the water generously and boil the potatoes over moderately high heat until very tender, about 20 minutes.
Drain the potatoes in a colander. Return them to the pot and shake the pot over moderately high heat until the potatoes are completely dry, about 1 minute. Remove from the heat.
Using a potato masher, crush the potatoes in the pot. Add 3/4 cup of the milk and the unsalted butter and season with salt and pepper. Continue mashing until the potatoes are smooth. Add the remaining 3/4 cup of milk and the truffle butter to the potatoes and mash until thoroughly blended. Season again with salt and pepper and serve piping hot.
Truffle butter is available at specialty food shops or by mail from Urbani USA.