Peel and chop the Russet or Yukon Gold potatoes into uniform chunks. Place them in a large pot, cover with cold salted water, and bring to a boil over medium-high heat. Cook for 15-20 minutes or until the potatoes are fork-tender. Once done, drain well and return them to the pot to steam off any excess moisture.

While the potatoes are cooling slightly, preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.

In a small saucepan, melt half of the unsalted butter over low heat. Once melted, gently warm the heavy cream in the same pot. Remove from heat and set aside.

Return the drained potatoes to the pot and mash them using a potato masher until smooth. Gradually mix in the melted butter, warm cream, grated Parmesan cheese, salt, and black pepper. Finally, add the egg yolks and stir until fully incorporated.

Transfer the creamy potato mixture into a piping bag fitted with a star tip. Pipe small mounds onto the prepared baking sheet.

Brush the tops with the remaining melted butter. Bake for 15-20 minutes, or until golden brown and crispy.

Remove from the oven and sprinkle with fresh chives or parsley and flaky sea salt. Serve immediately.