The wispy peaks and valleys of the homemade fluff give this side dish drama and intrigue that mini marshmallows (which is our traditional topping) just can’t. If you already have a favorite recipe for the base of the casserole, you can swap that in for mine here and just top your classic with the brown sugar fluff. —Ashley Rodriguez
Preheat your oven to 400° F. Place the yams on a baking sheet and roast in the oven until very tender, about 1 hour. Let cool, then remove skin with a paring knife. Cut yams into 1-inch chunks and place in the bowl of a food processor.
Process the yams until a smooth purée forms. To that, add the butter, 1/4 cup brown sugar, 1/2 teaspoon salt, cinnamon, and a few grinds of pepper. Pulse that together, then taste and adjust the seasoning.
Add the milk and the 2 whole eggs and process until smooth. Transfer the mixture to a baking dish.
Bake for 30 to 40 minutes, until slightly puffed.
For the marshmallow fluff: Make a double boiler by bringing a small pot of water to a simmer, then placing the bowl of your stand mixer or another large stainless steel bowl atop the water.
Add the 3 egg whites, remaining 2/3 cup brown sugar, and remaining 1/4 teaspoon salt and whisk continuously until the sugar dissolves and the liquid is warm to the touch.
Transfer the bowl from the stovetop to your stand mixer and use the whisk attachment to whip the whites on medium-high speed until it turns glossy with stiff peaks, about 5 to 8 minutes. (Use a hand-held mixer if you do not have a stand mixer.)
Once the casserole has finished baking, use a rubber spatula to cover the yam mixture with the fluff. Use the back of the spatula create dramatic peaks. This will give you a nice variation in color and texture once the dish is baked.
Return to the oven for 5 to 7 minutes, or until the marshmallow fluff starts to turn golden and the fluffy peaks are just shy of burnt.