Preheat your oven to 350°F (175°C).
Boil the sweet potatoes until tender.
Peel and mash the sweet potatoes in a large bowl.
Stir in sugar, melted butter, milk, eggs, vanilla, cinnamon, and salt until well combined.
Pour the sweet potato mixture into a greased baking dish.
Bake in the preheated oven for 25-30 minutes.
Remove from the oven and sprinkle mini marshmallows on top.
Return to the oven and bake until the marshmallows are golden brown, about 10 minutes.
Let the casserole cool slightly before serving.