Cut sweet potatoes in half lengthwise, then slice into ½" thick slabs and cut into fries.
Boil 2 quarts of water with 2 tablespoons of kosher salt and 2 tablespoons of white vinegar.
Add fries to boiling water and parboil for 6 minutes.
Drain fries and let cool.
Use a fork to scrape the sides of each fry to create grooves.
In a bowl, mix cornstarch with cold water to create a slurry.
Toss fries in the slurry to coat.
Preheat oven to 475°F and lightly coat a baking rack with oil.
Lay fries on the rack and lightly coat tops with oil.
Bake for 15 minutes, flip fries, coat again with oil, and bake for an additional 10-15 minutes until crispy.
Season fries to taste before serving.
