Preheat oven to 325°F.
In a large bowl, mix together the mashed potatoes, sugar, eggs, salt, butter, milk, and vanilla extract, until smooth. Pour in a greased 9×13" baking dish.
Mix the brown sugar and flour in a medium bowl and cut in the butter until the mixture is coarse. Stir in pecans, and sprinkle mixture over your sweet potato mixture.
Bake for 20 minutes. Sprinkle on marshmallows and bake an additional 10-15 minutes (or until lightly brown).