Preheat the oven to 450 degrees.
For the sweet potatoes: Poke each sweet potato a few times with a fork, wrap them individually in foil, and roast until tender, about 1 hour and 15 minutes. Remove from the oven to cool about 20 minutes. Leave the oven on, but reduce the temperature to 375 degrees.
Spray a 9x13 inch baking dish with cooking spray.
When the sweet potatoes are cool wnough to handle, peel them and place them in a large bowl. Break up the potaotes slightly with a fork. Add the sugar, cream, butter, milk, eggs, vanilla and salt. Using a handheld mixer, whip the potato mixture until well combined, 3-5 minutes.
Scrape the mixture into the prepared baking dish and spread it our evenly.
For the topping: In a medium bowl, mix together the cornflakes, pecans, brown sugar and melted butter. sprinkle the mixture evenly over the sweet potatoes and bake until golden brown, about 20 minutes. Turn the oven to broil and position the rack 4 to 5 inches from the broiler element.
Sprinkly the marshmallows on top of hte casserole and broil until marshmallows begin to brown and bubble, about 1 minute watching carefully. Serve hot.
Store in an airtight container in the refrigerator for up to 3 days. reheat in a 350 degree oven for 10 minutes or in a microwave for 3 to 5 minutes.