In a large bowl, mix together mashed sweet potatoes, 1/4 cup of melted butter, milk, brown sugar, vanilla extract and eggs. Add milk carefully, you may have to adjust the amount of milk to ensure the mixture isn't runny.
Spread sweet potato mixture into the prepared baking dish.
In a small bowl mix together the other 1/3 cup of brown sugar and flour.
Cut in the other 1/4 cup butter until the mixture is crumbly.
Stir in pecans.
Sprinkle pecan mixture over the sweet potatoes, covering it completely.
Bake for 25 minutes in preheated oven or until golden brown.
5 minutes before it's finished baking remove from oven and place marshmallows on top and continue baking until marshmallows are melted and browned.