Preheat the oven to 425ºF. Line a cookie sheet with parchment.
In a large pot, cover the potatoes with about an inch of cold water. Swish the potatoes around, then drain. Cover them with a fresh 1 inch of cold water. Bring to a boil over high heat and continue to cook until the potatoes are very tender and easily fall apart, 20-25 min, stirring occasionally (you can use a lid at first to speed up the boil)
2 lbs potatoes
Drain the potatoes very well, shaking excess water from the colander. Allow to sit and release steam in the colander for 5 minutes.
Return the potatoes to the pan off the heat. Using a potato masher or hand beaters, beat until very smooth. For very smooth potatoes, press them through a metal sieve with the back of a spoon or spatula (or use a ricer instead of mashing them).
Once smooth, add 2 tablespoons butter, cream, and spices. Stir with a spatula until combined. If they are still steaming, allow to cool another few minutes so you don’t cook the eggs.
4 tablespoon butter, ¼ cup heavy cream, 1 teaspoon kosher salt, ¼ teaspoon pepper, ¼ teaspoon garlic powder, 1/16 teaspoon nutmeg
Once just warm, add the egg yolks one at a time, mixing well between additions, until thoroughly combined.
3 egg yolks
Transfer the potatoes into a piping bag (or gallon zipper bag) fitted with a large star tip. On the prepared cookie sheet, make 12 swirls about 2.5 inches wide and 2 inches tall. Start with the inside of the circle and move outward until it is wide enough, then move back inward building up until it is tall enough.
If the potatoes seem a little saggy, refrigerate for 20 minutes or longer.
Before baking, gently brush with the remaining butter all over to help the edges get golden. Bake for 15-20 minutes, or until edges are golden. Cool for 5 minutes, then serve.