1. Heat oven to 400°F. Place sweet potatoes on a parchment paper-lined baking sheet; bake until soft, about 1 1⁄2 hours. Let cool.
2. Reduce oven temperature to 350°. Peel potatoes; pass them through a food mill into a bowl.
3. Whisk in evaporated milk, sugar, 4 tablespoons of the butter, vanilla extract, salt, allspice, and eggs. Transfer to a 2-quart oval baking dish.
4. In a food processor, pulse together cashews, brown sugar, flour, and remaining 2 tbsp. of butter until coarsely ground.
5. Crumble cashew mixture over half of casserole; top other half with marshmallows.
6. Bake until marshmallows are golden brown, about 30 minutes.

Serves 8 to 10.