Preheat your oven to 375°F.
Scrub the sweet potatoes under running water, then place them on a baking sheet.
Bake for 30-35 minutes until fork tender.
Slice open the baked sweet potatoes.
Scoop out the flesh into a large mixing bowl.
Discard the skins.
Add granulated sugar, milk, eggs, vanilla extract, and salt to the sweet potatoes.
Blend until smooth and creamy.
Transfer the sweet potato mixture into a greased 13-by-9-inch casserole dish.
Spread evenly across the dish.
In a separate bowl, mix the brown sugar, flour, and melted butter.
Stir in the chopped pecans until the mixture is crumbly.
Spread this topping over the sweet potato mixture and sprinkle marshmallows on top.