Preheat the oven to its lowest setting. Set a deep, heavy-bottomed Dutch oven fitted with a deep fry thermometer over medium-high heat, add oil to a depth of 2 inches, and set over medium-high heat to preheat. Line a large baking sheet with a wire rack and set it by the stove.
In a large bowl, whisk together the corn starch and 1 cup ice water. Add the sweet potatoes to the batter, tossing well to coat. (The batter should be quite loose and runny. Thin with a tablespoon or two more of water needed).
When the oil reaches 325°F, fry the sweet potatoes in batches: with your fingers spread far apart, lift the fries from the bowl, allowing most of the batter to drip away before transferring them to the oil to cook just until softened, 2–3 minutes. Use a spider strainer or slotted metal spoon to transfer the fries to the baking sheet while you continue blanch frying the rest of the potatoes in this manner.
Once all of the fries have been blanched, allow the oil to return to 350°F. Refry the potatoes in batches, stirring frequently and allowing them to crisp and brown, about 2 minutes more. Return the crisped fries to the baking sheet as they come out of the oil and hold in the warm oven while you continue cooking the rest of the fries. Season with salt and pepper and serve hot.