8 tablespoons (1 stick) unsalted butter, at room temperature
1/3 cup whole milk
2 large eggs
1 1/2 teaspoons pure vanilla extract
3/4 teaspoon kosher salt
1 cup cornflakes, crushed
1/3 cup chopped pecans
3 tablespoons light brown sugar
4 tablespoons (1/2 stick) unsalted butter, melted
2 cups mini marshmallows
Preheat the oven to 450°F.
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 15 minutes. Remove from the oven to cool, about 20 minutes. Leave the oven on, but reduce the temperature to 375°F.
Spray a 9 x 13-inch baking dish with cooking spray.
When the sweet potatoes are cool enough to handle, peel them and place in a large bowl. Break up the potatoes 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 to 5 minutes.
Scrape the mixture into the prepared baking dish and spread it out 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 an oven rack 4 to 5 inches from the broiler element.
Sprinkle the marshmallows on top of the casserole and broil until the 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°F oven for 10 minutes or in a microwave for 3 to 5 minutes.