Preheat the oven to 375 degrees F.
Bring a large pot of salted water to a boil. Reduce heat to medium-low and boil sweet potatoes until fork tender; about 20 minutes. Drain.
Mash the sweet potatoes with a potato masher until smooth. (Use a hand mixer if you want absolutely smooth with no lumps).
Stir in brown sugar, butter, cinnamon and salt until well combined. Add the milk a small amount at a time to reach desired consistency.
Spray an 11-inch x 7-inch (medium sized) casserole baking dish with nonstick cooking spray.
Spread sweet potatoes evenly in the prepared casserole dish and top with a layer of the marshmallows.
Bake for 20-25 minutes or until marshmallows are golden brown.
Allow to cool for 5-10 minutes before serving.
