Preheat oven to 375°F. Grease a 2 or 2 1/2 quart casserole dish and set aside.

Place the sweet potatoes in a large pot, cover with water, and boil until tender (about 7 minutes, depending on the size). Drain and return to the pot. Add butter, milk, cinnamon, brown sugar and salt to the potatoes and mash until smooth. Use a hand mixer, if desired, to make them extra smooth. Taste the potatoes and add additional seasonings, if desired. Transfer the mashed potatoes to the prepared casserole dish. Place in the oven until heated through (if they're warm it will only take a few minutes).

Remove the sweet potatoes from the oven. Top with the marshmallows. Place back into the oven for 8-12 minutes, until the topping is bubbly and brown (keep an eye on it so the marshmallows don't burn). You can also place it under the broiler very briefly to add a little more color to the marshmallows.