Boil the potato for 10 minutes, until just tender. Drain, then return the potatoes to the pan over very low heat for 1–2 minutes to dry out the potato. Transfer to a bowl and mash well.
Beat together the eggs, cream, parmesan and nutmeg. Season well, then add to the potato and mash well. Cover and leave for 20 minutes. Preheat the oven to 180°C (350°F/Gas 4).
Put the just-warm potato mixture in a piping bag with a 1.5cm (⅝inch) star nozzle. Pipe the mixture in swirls, not too close together, onto greased baking trays. Brush lightly all over with the extra egg yolk. Bake for 15–20 minutes, or until the potatoes are crisp and golden.