How to Make It
Preheat oven to 375°F. Coat a 12-cup muffin pan with cooking spray. Cut potatoes into 1⁄16-inch-thick slices using a mandoline or sharp knife.
Whisk together butter, olive oil, chopped thyme, chopped rosemary, kosher salt, and pepper in a large bowl. Add potato slices, and toss until evenly coated.
Working quickly, layer potato slices into stacks in muffin cups, filling each cup to the top. Bake in preheated oven until edges and tops are golden brown and centers are tender, 45 to 55 minutes. Remove from oven, and let potato stacks stand in pan 3 to 5 minutes. Carefully remove potato stacks from pan; sprinkle with sea salt, and, if desired, garnish with thyme sprigs. Serve immediately.