Preheat the oven to 375 degrees F. Line a baking sheet with parchment or a silicon baking mat.
Lay the boiled potatoes on a baking sheet and place into the oven until slightly dried on the surface, 10 to 15 minutes.
Remove the potatoes from the oven and process through a ricer or food mill. Allow to cool in a bowl for about 5 minutes.
Add the egg yolks, butter, a couple generous pinches of salt, a generous pinch of pepper, a pinch of nutmeg and 3/4 cup cream. Stir with a rubber spatula to combine. Taste and make sure the potatoes have been adequately salted.
Transfer to a large pastry bag and pipe through a large star tip in a circular/upward spiral motion onto the prepared baking sheet.
Make an egg wash by mixing the whole egg with the remaining 1/2 cup cream. Lightly brush the piped potatoes with the egg wash. (This is a little easier if you chill the piped potatoes for half an hour or so.)
Bake until golden brown around the edges. Serve on a pretty platter!