To make sweet potato casserole, preheat the oven to 350°F and line a baking sheet with foil. Bake washed and pricked potatoes for about an hour until soft. Peel when cooled. Spray a casserole dish with nonstick spray.
Mash potatoes and mix with sugar, vanilla, eggs, evaporated milk, and butter. Pour into the dish. For the topping, combine brown sugar, flour, and softened butter until crumbs form, then add pecans.
Sprinkle half of the marshmallows over the potato mixture, add the topping, and bake for 30 minutes. Afterward, add the remaining marshmallows and bake for an additional 10 minutes until golden brown. Allow to rest for 10 minutes before serving.