Preheat your oven to 350°F (175°C).
Boil the sweet potatoes in a large pot until tender, about 15-20 minutes.
Drain the sweet potatoes and return them to the pot.
Mash the sweet potatoes until smooth.
Stir in brown sugar, milk, melted butter, vanilla, cinnamon, and nutmeg.
Spread the mashed sweet potato mixture into a greased baking dish.
In a separate bowl, mix the flour, chopped pecans, and remaining brown sugar.
Sprinkle the pecan mixture evenly over the sweet potatoes.
Add mini marshmallows on top of the pecan mixture.
Bake in the preheated oven for 30-40 minutes, until the topping is golden brown.