Preheat oven to 350°F. Cover a baking sheet with aluminum foil and set aside.
Scrub the outside of each potato to remove any dirt, then poke potatoes several times on top with a fork (the larger the potato, the more holes you'll want). If desired, coat the outside of each potato with melted butter and salt.
Place potatoes on prepared baking sheet and bake for 60 minutes. You want them to be soft to the touch, so if they're still not soft at this point just leave them in longer until they are. The larger the potatoes, the longer they'll take to cook.
In a medium bowl, whisk to combine cinnamon and brown sugar. Set aside.
Once soft, slice each potato open vertically. Scoop out the inside of each potato (leaving enough against the flesh to keep it stable) and place it into your bowl of cinnamon and brown sugar. Stir to combine until mostly smooth with very few lumps - you can even use a hand mixer if you'd like.
Place the hollowed out potato skins back in the oven for another 10 minutes.
Remove skins from oven and fill with your prepared mixture. Top with marshmallows.
Bake 10 minutes and then broil just until the marshmallows turn golden brown.