Preheat your oven to 350°F (175°C).
Boil sweet potatoes in a large pot until tender (about 15-20 minutes).
Drain and mash the sweet potatoes in a large mixing bowl.
Add brown sugar, milk, melted butter, vanilla extract, and cinnamon to the mashed potatoes.
Mix until smooth and well combined.
Pour the sweet potato mixture into a greased 9x13 inch baking dish.
Evenly distribute the mini marshmallows over the top of the casserole.
Bake in the preheated oven for 30 minutes, or until marshmallows are golden.
Remove from the oven and let cool for a few minutes before serving.
Enjoy your delicious sweet potato casserole!