Make crispy sweet potato fries by cutting the potatoes into thin pieces, soaking them in cold water and tossing them in corn starch and oil. Bake the fries for 30 minutes on a foil-lined baking sheet. Turn the oven off, and let the fries cool slightly with the door ajar.
Cutting the sweet potatoes into 1/4-inch thin, matchstick-style fries ensures they cook evenly inside and outside. Place the cut fries into a bowl of cold water for about 45 minutes. Soaking helps remove some of the excess starch inside the fries so they crisp better. Drain the water, and pat the fries dry thoroughly.
Place the sweet potato fries in a large bag, and add about 2 tablespoons of cornstarch. Seal the bag, and shake until the fries are lightly coated. The cornstarch helps give the outside of the fries a crispy coating. Toss the fries in 1 tablespoon of cooking oil, which both aids in crisping and keeps them from sticking to the foil.
Spread the sweet potato fries onto an oiled, foil-lined baking sheet, making sure to leave space between the fries. Overcrowding causes steam, which makes the fries soggy. Bake the fries in the oven at 425 degrees Fahrenheit for about 15 minutes, then flip them over and continue cooking for another 15 minutes or until the fries are slightly browned. After turning the oven off, leave the fries in the oven with the door partially open for several minutes to further crisp the fries.