Preheat the oven to 425°F.
Bring a large pot of water to a boil and add 2 tablespoons kosher salt.
Add the potatoes, return to a boil, then reduce to a simmer and cook for 8 minutes.
Drain the potatoes and return them to the pot then cover with a lid and shake vigorously for about 5 seconds to rough up the edges.
Transfer the potatoes to a baking rack set over a sheet pan, arranging them in a single layer. Let them dry for at least 15 minutes.
Scoop the duck fat onto a sheet pan then place in the oven for 5 to 7 minutes until the fat is melted and very hot.
Carefully transfer the potatoes into the hot duck fat and gently toss to coat.
Roast for 25 to 35 minutes (depending the size of the potatoes), tossing halfway through, until the potatoes are browned and crispy.
Transfer to a serving platter and sprinkle generously with flaky sea salt.
Serve hot.