Preheat oven to 350 degrees F (175 degrees C).
In a large bowl, mix together mashed sweet potatoes, granulated sugar, eggs, milk, 1/4 cup melted butter, vanilla extract, and salt. Spread evenly in a 9x13 inch baking dish.
In a separate bowl, mix together brown sugar, flour, and 1/4 cup melted butter until crumbly. Stir in mini marshmallows. Sprinkle mixture over sweet potato mixture.
Bake for 30 to 40 minutes, or until topping is golden brown.