Wash and peel your sweet potatoes. Cut into 1/4 inch thick fries and rinse the potatoes for about 10-15 seconds under cold water then dry with paper towels or a clean dish cloth you don't mind turning slightly orange.
Mix the cornstarch with the water to make a slurry in a large bowl or large ziploc bag, add sweet potatoes and toss to coat.
Heat your oil in your pot to 350 degrees and add the fries 1 at a time so they don't stick together, but make sure you don't overcrowd your pan.
Fry for 1-2 minutes (no browning yet) and drain on a brown paper bag.
Once all the fries are done with their first dip in the oil, start the second round of frying, then fry them the second time for an additional 2 minutes, until they're browned and crispy.
To reheat: If you're making ahead for a crowd, you can heat them in the oven at 325 degrees for 5-7 minutes to crisp them back up.