Heat the oven to 400°F. Use about one tablespoon of butter to grease a large sheet pan with a rim.
Score the skin of each sweet potato with a paring knife, cutting four evenly spaced lines from tip to tip.
Cut into 1 ½-inch-thick rounds. Place the rounds on the sheet pan, cover tightly with foil or another baking pan, and roast for about 20 minutes, until soft when pierced with a paring knife.
Remove the pan from the oven, and set the oven on broil.
Use a fork to flatten each slice to about a ¾-inch-thick round.
Melt the remaining butter in a small pan; add garlic and rosemary, and stir for a few seconds.
Use a spoon to drizzle a little bit of the butter mixture over the sweet potato rounds.
Sprinkle with salt, pepper and Parmesan cheese, and place under the broiler for 2 minutes or so, until crispy and browned.
Serve hot.
