Preheat the oven to 425°F and line two baking sheets with parchment paper. Arrange two oven racks in the upper and lower thirds of the oven.

Slice the sweet potatoes lengthwise into 1/4- to 1/2-inch-thick planks. Slice the planks vertically into 1/4- to 1/2-inch-thick sticks. Slice any very long sticks in half crosswise. The smaller pieces will get crispier in the oven. Place the sweet potatoes in a large bowl.

In a small bowl, stir together the cornstarch and water until the cornstarch is dissolved. Pour the mixture over the sweet potatoes and add the garlic powder, paprika, and salt. Toss to coat, using your hands to make sure the spices are evenly distributed.

Evenly arrange the sweet potato fries on the prepared baking sheets, leaving a little space around each one. Drizzle generously with olive oil.

Bake for 15 minutes, or until blistering underneath. Remove from the oven and use tongs to flip the fries. If they look dry, drizzle with a bit more oil.

Return the pans to the oven, swapping their positions on the oven racks and rotating them front to back. Bake for another 7 to 12 minutes, or until the fries are browned and crisp.

Remove from the oven and let cool slightly. Toss with more garlic powder, paprika, and salt to taste and serve.