Light a campfire and let it burn down to embers. Nestle potatoes in embers and cook, rotating a quarter turn every 10 minutes or so, until very tender when pierced with a knife, 45–60 minutes. The potatoes will blacken and char on the outside; don't worry, you're only eating the flesh. If you prefer, wrap potatoes individually in foil and cook using the same procedure.
Using tongs, transfer potatoes to plates and let cool at least 10 minutes. Cut a slit lengthwise down each potato and mash flesh with a fork. Spread 1 tablespoon butter inside each potato and drizzle with maple syrup. Top with gingersnaps and marshmallows, if using, and serve.
