Preheat the oven to 400°F and coat a shallow baking sheet with nonstick spray.
Place the butter and olive oil in a small saucepan over low heat. Add the garlic, truffle salt, and pepper and heat until melted, stirring occasionally.
Scrub the potatoes thoroughly and cut a very thin slice along the length of each to allow them to sit flat while baking.
Being careful not to cut all the way through, use a thin-bladed, sharp knife, to cut the potatoes crosswise into thin slices (about 3/16-inch).
It's best to leave about 1/4-inch uncut at the bottom of the potatoes. If it helps, lay 2 wooden chopsticks on either side of the potatoes as you slice. This will prevent your knife from slicing all the way down to the cutting surface.
Arrange the potatoes on a baking sheet and brush the tops and sides with about 1/3 of the butter mixture.
Roast for 20 minutes, remove and brush with more of the butter mixture. Return to the oven for 20 minutes longer, remove again and brush with the remaining butter mixture.
Roast for an additional 20 to 30 minutes, or until the potatoes are tender and nicely browned on the outside.
Just before serving, brush with any butter that has accumulated on the baking sheet and sprinkle each potato with a little more truffle salt and a pinch of pepper.