Place the Potatoes in a large pot and cover them with cold water. Starting them in cold water allows the Potatoes to cook evenly from the outside to the center.

Bring the water to a boil, reduce the heat, and cook gently until the Potatoes are completely tender. A knife should pass through them without resistance.

Drain the Potatoes very well. Return them briefly to the warm pot so the remaining moisture can evaporate.

Rice the Potatoes - While the Potatoes are still warm, pass them through a potato ricer or food mill.

Add the Butter and Seasonings - Add the softened Butter, Salt and White Pepper. Mix gently until the Butter has incorporated and the Potatoes are smooth.

Cool the Potatoes Slightly - Allow the mashed Potatoes to cool slightly before adding the remaining ingredients.

Add the Egg Yolks and Nutmeg - Add the freshly grated Nutmeg and Egg Yolks. Mix thoroughly until the mixture is smooth and evenly combined.

Prepare and Fill the Pastry Bag - Twist the pastry bag immediately above the piping tip before filling it. Spoon in the Potato mixture, untwist the bag, and squeeze out any trapped air.

Pipe the Potatoes - Pipe the mixture onto the prepared baking sheet, leaving some room between each portion because they will expand slightly.

Chill Until Firm - Refrigerate the piped Potatoes for approximately 20 to 30 minutes, or until they become firm and hold their shape.

Preheat the Oven - While the Pommes Duchesse are chilling, preheat the oven to 425°F (220°C).

Bake Until Golden - Bake the chilled Pommes Duchesse at 425°F (220°C) for approximately 20 to 25 minutes, or until the ridges are lightly golden brown.

Cool and Refrigerate - If preparing them in advance, allow the baked Pommes Duchesse to cool completely so the steam can escape.

Butter, Reheat and Serve - Shortly before serving, gently brush the firm Potatoes with the Melted Butter. Return them to a 375°F (190°C) oven until heated through and beautifully glossy.