Preheat the oven to 425°F and line two large, rimmed baking sheets with parchment paper.
Cut the sweet potatoes into thin even strips, about ¼” in thickness all around; then transfer to a large bowl.
Sprinkle the cornstarch over the sweet potatoes and use your hands to toss together, making sure all the fries are well coated.
Divide the sweet potato strips between the two baking sheets, and leave behind any excess cornstarch in the bowl. Drizzle the olive oil on top and season with sea salt. Use your hands again to toss the potatoes with the olive oil and salt until there are no more visible streaks of corn starch.
Arrange the potatoes in a single layer, making sure not to overcrowd them. Bake for 20 minutes, then use a spatula to flip the fries and cook until they’re lightly browned and puffed, about 10-15 more minutes.
Remove from heat, add additional sea salt, if desired, and serve immediately with ketchup or your favorite sauce.