Preheat your oven to 350°F (175°C).
Peel and boil sweet potatoes in salted water until tender.
Drain and mash the sweet potatoes in a large bowl.
Add brown sugar, melted butter, milk, vanilla, eggs, cinnamon, and nutmeg to the mashed sweet potatoes.
Mix until well combined and smooth.
Pour the sweet potato mixture into a greased baking dish.
In a separate bowl, combine pecans and marshmallows (if using).
Sprinkle the pecan mixture evenly over the sweet potato layer.
Bake in the preheated oven for 30-35 minutes or until golden brown.
Allow to cool for a few minutes before serving.