Preheat the oven to 350°F
Peel and cut yams into chunks
Boil or steam the yams until tender
Drain the yams well
Mash the yams with butter, brown sugar, cinnamon, and a pinch of salt
Spread the mashed yams into a baking dish
Top evenly with marshmallows
Bake until the marshmallows are puffed and golden brown
Serve warm