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1
Preheat your oven to 400u00b0 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.
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2
Process the yams until a smooth puree 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.
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3
Add the milk and the 2 whole eggs and process until smooth. Transfer the mixture to a baking dish.
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4
Bake for 30 to 40 minutes, until slightly puffed.
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5
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.
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6
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.
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7
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.)
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8
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.
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9
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.
