1. 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.
2. 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.
3. 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.
Notes: Truffle butter is available at specialty food shops or by mail from Urbani USA (800-281-2330).