Peel potatoes and cut into ¼ in (6 mm) rings. Move rings to a medium-sized mixing bowl.
Add butter, syrup, salt, and pepper to the bowl of rings and toss to coat evenly.
Spray down a standard 12-cup cupcake tray with nonstick cooking spray and place sweet potato rings inside, stacking them up until level with the rest of the tray. Cover with aluminium foil.
Bake at 350˚F (180˚C) for 15 minutes. Remove foil, then bake the potatoes uncovered for 5 additional minutes.
Once cooked, remove tray from the oven and quickly add marshmallows, placing 1 marshmallow on top of each stack of potatoes. Turn oven to broiler setting and place the marshmallow topped stacks under the heat for 1 minute.
Using a fork or spoon, carefully remove stacks from baking tray and transfer to plate for serving.