Preheat the oven to 425°F.
Prepare baking sheets with parchment paper and then lightly spray with cooking oil. Set aside.
Peel the potatoes and then cut them evenly so they’re all roughly the same size, but not too small.
Add the chopped potatoes to a large pot with cold water covering 2” above the potatoes.
Add the salt to the water and bring to a boil over medium heat. Reduce the heat to a rolling boil and cook for about 15-20 minutes, until the potatoes are tender.
Using a slotted spoon transfer the potatoes to the prepared baking sheet and place them in the oven for 10 minutes.
Once baked, transfer the potatoes to a large mixing bowl and mash with a potato masher.
Add 5 tablespoons of the butter, 1 cup of the parmesan, the black pepper and garlic powder. Use a stand mixer with the wire whip, whip the potatoes and other ingredients on a medium speed for about 5 minutes. Scrape down the sides intermittently.
Add the egg yolks, one at a time while the whip is moving on low speed. Once they’re completely mixed turn off the mixer.
Transfer a portion of the potatoes to a piping bag with a large star (or 4B) tip. You can use a glass to hold the bag while you fill it to help avoid any mess.
Pipe the potato mixture into nice spiral mounds, about 3” wide and 2” tall on a clean, parchment-lined baking sheet.
Place the baking sheet, uncovered, into the freezer for about 15-20 minutes.
Melt the remaining 3 tablespoons of butter.
Remove the baking sheet from the freezer. Brush the melted butter lightly over the potato swirls.
Sprinkle the remaining parmesan cheese over the tops.
Bake in the preheated oven for about 15-20 minutes, depending on your oven. Watch the potatoes close, you can remove them once they start to brown on the edges.