How to Make It
Preheat oven to 350°F. Wrap potatoes individually in foil. Arrange on a baking sheet. Roast at 350°F until very tender, 60 to 75 minutes. Set aside until cool enough to handle, about 15 minutes. Peel potatoes; discard skins. Place potatoes in a large bowl; mash until smooth. Cool completely, about 20 minutes. Stir in milk, butter, vanilla, salt, pepper, and egg yolks. Spread mixture evenly in a 2-quart glass or ceramic baking dish coated with cooking spray.
Whisk egg whites in a medium bowl until frothy. Stir in walnuts, oats, brown sugar, and cardamom. Sprinkle evenly over sweet potato mixture. Bake at 350°F, uncovered, until top is toasted and edges are bubbling, 40 to 50 minutes.