Preheat your oven to 180°C/350°F.
Peel the sweet potatoes and slice off both ends. If the potatoes are wider than the muffin tray cups, shave them down to the size of the muffin cups using a peeler. Thinly slice into rounds about 3mm (1/8 inch) thick.
Brush the inside of the muffin tray cups with melted butter. In a large mixing bowl, combine the remaining melted butter with the grated garlic, rosemary, salt, and black pepper. Toss the sweet potato slices in the butter mixture until well coated.
Place one sweet potato slice into each muffin cup. Toss the remaining slices in Parmesan cheese until well coated, then stack the slices in each muffin cup, creating small towers.
Use a brush to collect any remaining butter or Parmesan mixture from the bowl and brush it over the sweet potato stacks. If the Parmesan mixture is too clumpy, loosen it with a little oil.
Cover the muffin tray with foil and bake in the oven for 35 minutes.
After 35 minutes, remove the foil and increase the oven temperature to 200°C/400°F. Bake for another 15 minutes until the stacks are lightly charred and soft in the center (test with a butter knife).
Use a spoon to carefully remove the sweet potato stacks from the muffin tray, and drizzle or brush them with any remaining butter from the tin. Serve and enjoy!