Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.
Boil potatoes in salted water until tender, about 15-20 minutes. Drain well.
Mash the potatoes until smooth. Stir in heavy cream, butter, Parmesan, salt, pepper, and nutmeg.
Let the mixture cool slightly, then mix in 2 egg yolks until fully combined.
Transfer the mixture to a piping bag fitted with a star tip.
Pipe 3-inch wide swirls onto the baking sheet.
Brush each swirl gently with the remaining egg yolk.
Bake for 18–20 minutes, or until golden and lightly crispy on the edges.