Preheat the oven to 350 degrees F (175 degrees C).
Place sweet potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 15 minutes. Drain.
Place 1/2 of the sweet potatoes in a medium mixing bowl. Mash, then mix in condensed milk, butter, and cinnamon. Add remaining sweet potato cubes and sprinkle in all but 1 tablespoon brown sugar; stir gently to combine but not so much that the sugar dissolves. Transfer potato mixture to a 2-quart baking dish, top with marshmallows, and sprinkle remaining brown sugar on top.
Bake in the preheated oven until marshmallows are golden brown, 10 to 15 minutes.
Source: Read Full Article