Preheat oven to 400 degrees. Place the sweet potatoes on a parchment-lined baking sheet and roast in the oven until very tender. The amount of time will depend on the size of your sweet potatoes. This could take 35 minutes or up to an hour. Remove them from the oven when you can squeeze them with your oven mitts.

Lower the oven temperature to 375 degrees. Grease an 8 x 8-inch or 9-inch round baking dish with butter or oil.

When the sweet potatoes are cool enough to handle, peel and discard the skin. Place the sweet potatoes in a food processor with the eggnog, egg whites, salt, nutmeg or cinnamon, and vanilla. Process until smooth. Transfer the mixture into the prepared baking dish and smooth the top with a spatula. At this point, the casserole can be made up to 2 days in advance and kept covered in the refrigerator.

Place all the topping ingredients in the bowl of a food processor and pulse until finely chopped. Topping may be made up to 3 days in advance and kept in an airtight container in the refrigerator.

Sprinkle the sweet potato mixture evenly with the topping. Bake uncovered for 30 minutes or until the top is golden and crispy and the casserole is hot. Leftovers are delicious for breakfast cold, too!