Preheat oven to 400°F. Line a sheet pan with foil.
Pierce the potatoes all over with a fork.
Bake for 30-50 minutes, depending, on the size of your sweet potatoes, or until the potatoes fork tender. You can tell if they're fork tender because the fork will easily go through the potatoes.
Carefully cut the potatoes in half, and allow them to cool.
Decrease the oven temperature to 350°F.
In a medium bowl, whisk together the sugar, butter, milk, eggs, vanilla, salt, and cinnamon.
Scoop the flesh of the potatoes, and add it into the bowl.
Mash everything together.
Spread the mixture into a medium (2 quart) casserole dish.
Bake for 25-30 minutes until heated though.
Top the potatoes with the mini marshmallows and return to the oven to bake for an additional 10 minutes, or until the marshmallows are golden brown.
Notes
Sweet potatoes - Most of the yams sold in the U.S. are actually sweet potatoes. Feel free to use "yams" if you can't find sweet potatoes.
Brown sugar - Light or dark brown sugar will work. Whether or not you should pack the brown sugar depends on your desired sweetness.
Pack the brown sugar for a sweet casserole.
Butter - I like to use salted butter. Feel free to use unsalted butter if you prefer.
Milk - Any milk from skim to whole milk will work. Whole milk will give you a creamier casserole.