Preheat the oven to 400F and have a 12-cavity muffin tin at the ready. If you do not have a muffin tin, see notes for an alternative way of cooking the stacks.
Peel the yams and cut off one of the tips on each so that it exposes a 2-inch slice to start.
Carefully use a mandolin slicer or a sharp knife, to cut very thing slices. Set the setting on the mandolin to 1/16" if possible. The thinner the slices, the crispier the edges will become.
If the slices are too wide for the muffin tin, you can either, cook them with the alternative pan method, or use a 2" round cookie cutter to create uniform slices.
Place the melted butter in a large bowl. Add the crushed garlic, parmesan, rosemary, and salt to the butter.
Add the prepared sweet potato slices to the butter mixture and toss them, coating each piece. Using your fingers to separate and toss each thin slice works very well here.
Create 12 even stacks of the coated slices and place them in the muffin tin. The stack might be higher than the muffin tin, but they will shrink while baking.
Cover the muffin tin with foil and place the stacks in the preheated oven. Cook for 30 minutes. Remove the foil and cook for another 25 minutes.
Take the roasted sweet potato stacks out of the oven. Right away, loosen up the stack and remove them from the tin, flipping them over. Serve them hot.