Preheat the oven to 350°F (175°C).
Boil sweet potatoes in a large pot until tender, about 15-20 minutes.
Drain and mash the sweet potatoes in a large bowl.
Add brown sugar, milk, melted butter, eggs, vanilla extract, cinnamon, and nutmeg to the mashed sweet potatoes and mix well.
Transfer the mixture to a greased baking dish.
In a separate bowl, combine chopped pecans and remaining brown sugar, then sprinkle over the sweet potato mixture.
If desired, add mini marshmallows on top.
Bake in the preheated oven for 30-35 minutes, or until heated through and the topping is golden brown.