Preheat the oven to 375°F.
Wash and dry potatoes, prick all over with a fork. Roast the potatoes on a baking tray until tender, roughly 75-90 minutes.
Remove potatoes from skins and mash the sweet potatoes in a bowl.
Add the goat butter and smoked brown sugar. Mix well to thoroughly combine. Place in a serving bowl and top with marshmallows, if using. Toast marshmallows with a torch. Serve right away.