1-Peel and chop the potatoes: Peel and chop the potatoes. Transfer them to a large pot and cover with cold water. Add a generous pinch of salt and bring to a boil over medium-high heat. Cook for about 15-20 minutes until fork-tender.
2-Preheat the oven to 425°F and line: Preheat the oven to 425°F and line a baking sheet with parchment paper.
3-While the potatoes are cooking, melt the: While the potatoes are cooking, melt the butter in a small bowl in the microwave. Warm the cream in a microwave-safe bowl. Set both aside.
4-Drain the potatoes thoroughly and return them: Drain the potatoes thoroughly and return them to the pot over low heat to help evaporate any excess moisture. Mash the potatoes using a potato ricer or masher until they are smooth and free of lumps.
5-Add two tablespoons of the melted butter,: Add two tablespoons of the melted butter, the warm heavy cream, grated Parmesan (if using), salt, and pepper. Mix well, then stir in the egg yolks until well combined.
6-Transfer the mashed potato mixture to a: Transfer the mashed potato mixture to a piping bag fitted with a large star tip (to create the swirls). Pipe the potatoes into small mounds (or rosettes) onto the baking sheet, spacing them about 1 inch apart.
7-Brush the potato mounds with the remaining: Brush the potato mounds with the remaining melted butter and bake for 15-20 minutes, or until the Duchess Potatoes are golden brown on the edges and slightly crispy.
8-Remove from the oven and garnish with: Remove from the oven and garnish with fresh chopped chives or parsley. Sprinkle the sea salt over the top, serve immediately, and enjoy.