Preheat your oven to 350°F (175°C). Peel the sweet potatoes and cut them into chunks. Place them in a large pot, cover with water, and bring to a boil. Cook until tender, about 15-20 minutes. Drain and let cool slightly.
In a large mixing bowl, mash the cooked sweet potatoes until smooth. Add the brown sugar, milk, melted butter, vanilla extract, cinnamon, nutmeg, and salt. Mix well until all ingredients are combined and the mixture is creamy.
Pour the sweet potato mixture into a greased 9x13-inch casserole dish, spreading it evenly.
In a separate bowl, combine the mini marshmallows, chopped pecans, brown sugar, melted butter, and cinnamon. Mix until the pecans are well coated.
Sprinkle the pecan and marshmallow topping evenly over the sweet potato filling.
Place the casserole in the preheated oven and bake for 25-30 minutes, or until the topping is golden brown and the marshmallows are melted and slightly toasted.
Remove from the oven and let it cool for a few minutes before serving.