Preheat the oven to 425ºF/218ºC.
Transfer the peeled and cubed potatoes to a large bowl of cold water and soak for 5 minutes. Then agitate the potatoes by swishing them around in the water with your hands, then drain. The water will be very starchy (this is the starch washing off). Rinse the potatoes again in cold water. Drain the potatoes, then wrap in a clean kitchen towel or paper towels and dry well.
In a small bowl, combine the cornstarch and several rounds of freshly cracked black pepper.
Transfer the potatoes to a parchment paper-lined baking sheet, and use your hands to coat the potatoes with the oil. Sprinkle the cornstarch mixture over the potatoes, then toss again with your hands until they’re evenly coated. Spread the potatoes out in an even layer with no overlap.
Roast in the oven for 18 to 20 minutes until golden on the bottom, then remove from the oven and flip the potatoes. Return to the oven and roast for another 15 minutes, until browned and crispy. If they're not as browned as you’d like, roast for another 5 minutes. Note: The exact timing of the potatoes will depend on how small you've cut them.
Remove the potatoes from the oven and sprinkle generously with kosher salt or sea salt to taste. Allow to rest for a few minutes before using or eating.