Preheat oven to 425°F (220°C) and line a baking sheet with parchment paper.
Boil potatoes in salted water until tender, about 15–20 minutes.
Drain and mash the potatoes until smooth. Let cool for 5 minutes.
Stir in butter, egg yolks, cream, salt, pepper, and nutmeg until smooth and creamy.
Transfer mixture to a piping bag with a star tip and pipe swirls onto prepared baking sheet.
Brush tops with melted butter or sprinkle with Parmesan, if desired.
Bake for 18–20 minutes or until golden brown. Serve warm, garnished with parsley if using.