Preheat oven to 400 degrees F (200 degrees C). Grease a baking sheet.
Fill a 3-quart pot with 4 inches water and 1 teaspoon salt. Add potatoes; bring to a boil. Reduce heat to low; simmer until tender, 20 to 25 minutes. Drain; return potatoes to the pot.
Cook potatoes over low heat until remaining water steams off, about 3 minutes. Let cool, at least 15 minutes.
Place potatoes in a food processor and process until smooth. Let stand for 5 minutes. Add butter, egg yolks, remaining 1 teaspoon salt, nutmeg, and pepper. Mix until just combined; spoon into the piping bag.
Pipe potato mixture onto the prepared baking sheet into eighteen 2-inch rosettes with a circular motion, finishing with a small peak in the center. Refrigerate until slightly firmer, about 25 minutes.
Mix egg and heavy cream together in a bowl; brush mixture onto the rosettes.
Bake in the preheated oven until golden brown, 35 to 40 minutes.
Use a potato ricer instead of a food processor if preferred.
If timing is an issue you can shorten the process; skip the cooling of the rosettes and spoon on some melted butter instead of using the egg wash. This may flatten the rosettes a bit; the egg wash is much more visually appealing.
Per Serving: 165 calories;7.9 g fat;
20.7 g carbohydrates;
3.9 g protein;
91 mg cholesterol;
539 mg sodium.