Place potatoes in a large pot and cover with cold salted water. Bring to a boil, then reduce to a simmer and cook until tender, about 15-20 minutes. Drain well.
Return potatoes to the pot and cook over low heat for 1-2 minutes to dry them out. Mash until smooth, or pass through a ricer.
Add butter, egg yolks, warm cream, salt, white pepper, and nutmeg. Mix until smooth and well combined. Let cool slightly.
Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Transfer potato mixture to a piping bag fitted with a large star tip. Pipe rosettes (about 2 inches wide and 1.5 inches tall) onto the prepared baking sheet, spacing 1 inch apart.
Brush each rosette with beaten egg for a shiny, golden crust.
Bake for 20-25 minutes until puffed and golden brown. Serve immediately.
Tips
For extra crispy rosettes, chill piped potatoes for 15 minutes before brushing with egg wash.
Leftover rosettes can be reheated in a 350°F oven for 5-7 minutes.
If you don't have a piping bag, use a zip-top bag with a corner snipped off.