Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.
Place potatoes onto prepared baking sheet. Place into oven and bake until tender, about 1 hour. Let cool before peeling.
Reduce oven temperature to 350 degrees F. Lightly oil a 9-inch pie plate or coat with nonstick spray.
To make the topping, combine cornflakes, pecans and brown sugar. Stir in butter until well combined; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat sweet potatoes, butter, sugar, milk, eggs, vanilla and salt on medium-high until well combined, about 1-2 minutes.
Add sweet potato mixture to prepared baking dish. Sprinkle with cornflakes mixture around the outside of the dish, leaving room in the center.
Place into oven and bake until the topping is golden, about 20 minutes. Add marshmallows to the middle and bake until lightly golden on top, an additional 5 minutes.