1. Preheat oven to 450°.
2. Combine 2 teaspoons thyme and kosher salt in a small bowl.
3. Drizzle butter into a 10-inch cast-iron skillet coated with cooking spray. Arrange a layer of slightly overlapping potato slices in a circular pattern in pan; sprinkle with about 1/2 teaspoon salt mixture and drizzle with 1/2 teaspoon truffle oil. Repeat layers 4 times, ending with truffle oil. Press potato mixture firmly to pack. Cook over medium-high heat for 6 minutes without stirring.
4. Cover with foil, and bake at 450° for 20 minutes on bottom rack in oven.
5. Uncover and bake an additional 20 minutes or until potatoes are tender when pierced with a knife. Loosen edges of potatoes with a spatula or knife. Place a plate upside down on top of pan; invert potatoes onto plate. Sprinkle with remaining 1/2 teaspoon thyme.