Square off the potatoes by trimming off the edges until you have a square shape. Cut the potato into chip shapes that are roughly 2 x 6cm.
Place the cut chips into a bowl of cold water and rinse well until the water turns cloudy. This is the starch that has been removed from the potato. Drain into a colander and rinse again under cold water.
Place the chips into a large pot and cover with cold water. Season with a generous amount of salt. Place the pot onto the stove on a high heat and start the timer for 20 minutes as soon as you turn the heat on. Once the water is boiling, turn the heat to medium. Cook the potatoes until they are fork tender. Drain and place the chips onto a cooling rack to allow all the moisture to drip off. Place the cooling rack into the fridge or freezer until the chips are cold, not frozen. 30 minutes in the freezer and up to 2 hours in the fridge.
Fill a large pot with vegetable or grapeseed oil and bring to 140 degrees Celsius. Using a slotted spoon, carefully place the chips into the hot oil and fry for 8 minutes. Using a slotted spoon, remove the chips from the oil and place back onto the cooling rack. Place the chips back into the fridge or freezer until cold.
For the second fry, use the same oil but heat it to 180 degrees Celsius this time. Using a slotted spoon, carefully place the chips into the hot oil and fry for 4 - 6 minutes or until golden and crispy. Using a slotted spoon, remove the chips from the oil and place back onto the cooling rack. Season generously with flakey sea salt and serve.