Heat your oven to 350°F. Butter your 9-by-13-inch baking dish.
On a baking sheet, bake the sweet potatoes at 400°F until fork-tender (about 45 to 60 minutes).
Let the potatoes cool enough to handle, remove the skins, and place the flesh into a large bowl.
Mash the sweet potatoes with the butter, brown sugar, vanilla, salt, cinnamon, nutmeg, milk, and beaten eggs until blended.
Spread the sweet potato mixture evenly into the prepared baking dish. Add pecans if using, then cover the top in an even layer of mini marshmallows.
Bake at 350°F for 25 to 30 minutes, until the center is set and the marshmallows turn light brown. (Watch closely so they do not burn).
Let the casserole rest for 10 minutes before serving.
