Preheat oven to 425°. Place sweet potatoes in a large bowl and cover with cold water. Let soak at least 30 minutes, then drain and dry well with paper towels.
On two large baking sheets, toss sweet potatoes with oil, cornstarch, and garlic powder and season generously with salt and pepper. Bake until crispy, 40 to 45 minutes, tossing halfway through.
