1. Preheat the oven to 350F. Grease a 2-quart baking dish.
2. Mash the sweet potatoes in a large bowl. Stir in the sugar, butter and nutmeg.
3. Shape the mixture into 8 balls and press a marshmallow into each. Spread out the cornflakes in a shallow bowl. One at a time, dredge the puffs in the cornflakes to coat.
4. Flatten the bottom of each puff slightly so they will be stable when baking, then arrange in the prepared baking dish. Bake until lightly browned, 10 to 15 minutes.
Variation
For a casserole presentation, add 1/4 to 1/2 cup milk while mashing the sweet potatoes. Spoon into a greased 9-inch-square baking dish. Sprinkle with cornflakes, top with the marshmallows and bake for 20 to 30 minutes.