1. Preheat oven to 375F (190C). Grease 12 muffin cups with butter and set aside.
2. Peel sweet potatoes, then, using a mandolin or a knife, thinly slice sweet potatoes.
3. Place sliced sweet potatoes in a large bowl, add melted butter, olive oil, grate parmesan cheese, thyme, salt and pepper. Toss to coat evenly.
4. Stack sweet potato slices in muffin cups.
5.Bake for about 40 minutes, until edges and tops are golden brown, and potatoes are tender.
6. Allow to cool for 5 minutes, then remove the cups, serve hot/warm.