Place the potato chunks in a large pot and cover with cold water by about 2 inches. Add the tablespoon of kosher salt. Bring to a boil over high heat, then reduce to a steady simmer and cook for 15 to 18 minutes, until a fork slides into the largest piece with no resistance at all. Drain thoroughly in a colander and let the potatoes sit for 2 minutes so the steam escapes. Wet potatoes will ruin the texture of the rosettes.

While the potatoes are still hot, pass them through a potato ricer or fine-mesh food mill back into the pot or into a large mixing bowl. Do not use a food processor or blender, which will make the potatoes gummy. Add the butter cubes right away and stir until melted and fully incorporated.

Let the potato mixture cool for 4 to 5 minutes, stirring occasionally, until it is warm but not steaming hot. This is important before adding the egg yolks because if the potatoes are too hot they will scramble the eggs. Stir in the warm heavy cream, Parmesan, egg yolks, fine salt, white pepper, and nutmeg. Mix until the mixture is very smooth and holds a shape when you drag a spoon through it.

Preheat the oven to 400 degrees F. Line two large rimmed baking sheets with parchment paper. Fit a large piping bag with a 1M or 2D star tip and fill it two-thirds full with the potato mixture. Twist the top closed and pipe rosettes onto the prepared pans by holding the bag straight up, squeezing steadily from the base upward, then releasing pressure and pulling straight up to finish the tip. Space them about 1 1/2 inches apart. Each rosette should be roughly 2 1/2 inches wide.

Lightly brush or spray the outside of each rosette with a small amount of olive oil or melted butter. This helps the ridges turn golden in the oven. If you are making these ahead, cover loosely with plastic wrap and refrigerate at this point for up to 18 hours. When ready to bake, remove from the fridge and proceed without thawing.

Bake for 22 to 25 minutes, rotating the pans halfway through, until the tips and ridges are deep golden brown and the bottoms have a thin crust. The edges should be set and the potatoes should not jiggle when you nudge the pan. If they were refrigerated overnight, add 2 to 3 extra minutes.

Remove from the oven and let the rosettes sit on the pan for 2 minutes before carefully transferring them to a serving platter with a thin spatula. Sprinkle with sliced chives and a little extra Parmesan. Serve warm.