Peel the potatoes completely, then cut them into even 1/2-inch cubes.
Put the potato cubes in a large pot and add enough water to cover them by at least 3 inches. Bring the water to a boil over high heat, then reduce the heat enough to maintain a controlled, gentle boil. Start the timer once the water is boiling and cook the potatoes for a full 8 minutes. At 8 minutes, the outside of a cube should be tender while the center still offers slight resistance and the cube holds its shape.
Drain the potatoes completely and discard the cooking water. Let the cubes steam-dry in the colander for 5 to 10 minutes. Blot away any surface moisture if needed. The drier the potatoes are, the better the crispier they will turn out.
Heat a 12-inch cast-iron or other heavy skillet over medium to medium-high heat. Add the duck fat and let it melt until it shimmers but does not smoke.
Add the potatoes in a single layer. If they do not fit without crowding, cook them in two batches. Leave them undisturbed for about 5 minutes, then turn them in sections. Continue cooking for 8 to 10 minutes, turning only as needed, until the potatoes are browned and crisp. Reduce the heat to medium if the outside is browning before the centers finish.
During the final 2 to 3 minutes, sprinkle the potatoes with the sea salt, garlic powder, and white pepper. Turn gently to coat without breaking the cubes. Taste and adjust the salt.
Serve right away while the edges are crisp.