Preheat the oven to 450 degrees.

Cut 1 stick unsalted butter into small cubes. Peel 3 ½ to 4 pounds russet potatoes. Cut into chunks that are quite large, about 2- to 2 ½ inches. Transfer to a large pot, cover with water, and salt very generously. Place over high heat and bring to a boil. Cook for 4-6 minutes, stirring gently until the centers of the potatoes are still firm when pierced with a toothpick.

Drain the potatoes, return them to the pot, and place over medium heat, shaking the pan until all excess moisture evaporates. Turn off the heat, cover with a lid, and vigorously shake the pot to rough up the exteriors of the potatoes. Add the cubed butter, let melt, and gently fold to coat the pieces.

Evenly divide potatoes between 2 rimmed baking sheets, scraping all the mashed bits from the sides of the pot. Season generously with salt and transfer to the oven. Roast, flipping potatoes and rotating the sheet pans twice during cooking, until beautifully golden on all sides, about 35-50 minutes.

Thinly slice the whites and light green parts of 1-2 leeks to measure 1 ½ cups. Coarsely crush 3 tablespoons pink peppercorns or 1 tablespoon black peppercorns. Transfer peppercorns to a saucepan, along with 2 cups heavy cream, 3 tablespoons cognac, and the sliced leeks. Bring to a boil, reduce heat to maintain a steady simmer, and let reduce to the consistency of melted ice cream, about 5-7 minutes. Season with salt.

Thinly slice 1 small bunch chives. Rewarm potatoes and Au Poivre cream just before serving, adding a splash of heavy cream to loosen if necessary. Spoon some of the Au Poivre sauce onto a serving platter and scatter the potatoes and chives on top. Serve with the remaining sauce and more pink or black pepper.