Preheat oven to 425°F. Line a baking sheet with parchment paper.
Boil potatoes in a large pot of salted water until fork tender.
2 1/2 pounds Yukon gold potatoes
While the potatoes cook, heat 6 tablespoons butter, garlic, and cream in the microwave until butter is melted.
8 Tablespoons salted butter, 4 garlic cloves, 1/4 cup heavy cream
Drain potatoes well then press the boiled potatoes through a potato ricer or mash with a potato masher.
Add Parmesan cheese, nutmeg, butter, garlic, and milk mixture. Season with salt and pepper, to taste. Stir in egg yolks.
1 1/2 teaspoons kosher salt, 1/2 teaspoon freshly ground black pepper, 1/2 cup finely shredded Parmesan cheese, 1/8 teaspoon ground nutmeg, 4 egg yolks
Pipe into 12 swirled mounds by transferring the mashed potato mixture to a piping bag fitted with a large star tip (I used an Ateco 826). They can also just be spread in a baking dish. If making ahead, you can freeze the potatoes until firm and then transfer to an airtight container until ready to bake.
Brush with remaining melted butter. Bake for 18-20 minutes. If baking direct from frozen, add an addition 5 to 10 minutes to the bake time. After baking, garnish with fresh parsley and additional Parmesan cheese before serving.
Chopped fresh parsley, Additional grated Parmesan cheese