Preheat oven to 400 degrees and line one or two large baking sheets with parchment paper. Peel and cut sweet potatoes into sticks about 1/4-inch wide.
In a large mixing bowl, combine cut sweet potatoes and cornstarch. Toss to give the potatoes a thin coating, then pour them out into a colander and shake it over the sink to remove extra starch. Wipe out the bowl with a paper towel, return the potatoes to it, and add the oil, garlic powder, chili powder, and paprika. Toss well.
Spread potatoes out on the baking sheets, leaving as much space between them as possible. Roast for 30 minutes, using a spatula to flip them over halfway through.
If you have time, turn off the oven and leave the fries inside for an additional 20-30 minutes while the temperature drops. Remove, sprinkle with sea salt, and enjoy!
Notes
This recipe yields 6 moderate side portions, but I often find they disappear so quickly that I round up the fries per person.