Preheat the oven to 400ºF.
Put the Yukon Gold potatoes in a large saucepan, cover with water, season with salt, and bring to a boil over high heat. Reduce the heat to medium and simmer until tender, 8 to 10 minutes. Remove from the heat and drain. Return the potatoes to the saucepan and, over low heat, let the potatoes dry for about 1 minute. Add the butter, heavy cream, and truffle oil. With a hand mixer, mash and stir the mixture until almost smooth, leaving a few small lumps. Season with salt and pepper.
Using a mandoline, if you have one, cut the Idaho potatoes into shoestrings. (If you don’t have a mandoline, simply coarsely grate them.) Rinse under cold water and pat dry. In a large mixing bowl, toss the potatoes with the olive oil and season with salt and pepper.
Heat the pan over medium heat, then add the shoestring potatoes, spreading them evenly on the bottom of the skillet, then, using your fingers, press them up the sides of the skillet about 2 inches. Cook over medium heat until golden brown, 5 to 6 minutes. Flip the potatoes over andcontinue to cook until golden brown.
Gently transfer the potato pancake to a baking sheet lined with parchment paper. Spoon the mashed potatoes evenly over the potato crust, leaving a 1/2-inch border. Sprinkle with the cheese. Place in the oven and bake until the cheese has melted and the potatoes are heated through, about 5 minutes.
Shave the truffles over the tart. Carefully slide the tart onto a serving platter, garnish with the chives, then slice into wedges to serve.
Yield: 8 servings