Peel the sides of the potatoes but leave some skin on the ends. As you peel, place the potatoes in a bowl filled with cool water. Cut the potatoes into matchsticks (1/8-inch square) and place back into the bowl of clean cool water. Allow the potatoes to sit for 1 hour, changing the water as it becomes cloudy to remove the starch.
Heat the canola oil in a large, heavy Dutch oven or deep skillet to 325 degrees F.
Drain the potatoes and dry well with paper towels. Carefully add the potatoes to the oil, in batches if necessary, and cook until they just lose their opaque look and are starting to firm up, 5 to 6 minutes. Drain well on paper towels.
Heat the duck fat in a heavy saucepan to 375 degrees F; ideally you want the duck fat 3 to 4 inches deep.
Carefully add the fries to the duck fat and cook until crispy and evenly golden, about 8 minutes. Remove to a paper towel-lined plate and season with the sea salt or truffle salt.
Cook's Note: Strain the duck fat through a fine mesh strainer and transfer to a glass container. Keep refrigerated or frozen for up to 6 months.
Recipe courtesy of Guy Fieri