Image credit to: Boston Organics
Preheat oven to 400 degrees. Roast sweet potatoes over an aluminum foil-lined baking sheet for approximately one hour, or until tender. Cut in half and allow to cool until able to touch. Peel and mash sweet potatoes (this should yield a scant 5 cups).
Lower oven temperature to 350 degrees.
In a stand mixer, combine mashed sweet potatoes and maple syrup (adjusting to personal taste and for the sweetness of the potatoes. Keep in mind that marshmallows are very sweet, too.) Add butter, half and half, eggs, cinnamon, and salt until well combined. Pour mixture into a 10-inch cast iron skillet. Bake for 30 minutes.
Remove casserole from the oven and increase heat to BROIL. Top the casserole with marshmallows. Cook, WATCHING CAREFULLY, until the marshmallows just begin to melt and brown, about 1 - 2 minutes.