Adjust oven rack to lowest position and heat oven to 450 degrees.
Place potatoes in bowl, wrap tightly with plastic, and microwave until translucent around edges, 3 to 5 minutes, shaking bowl to redistribute potatoes halfway through cooking. Transfer potatoes to cooling rack and thoroughly blot fry with paper towels.
Coat rimmed baking sheet with 5 tablespoons oil. Transfer to oven and heat until just smoking, 5 to 7 minutes. Meanwhile, whisk remaining oil, cornstarch, and salt in large bowl. Add potatoes to bowl and toss to coat.
Arrange in a single layer on hot baking sheet and bake until deep golden brown and crisp, 25 to 35 minutes, flipping potatoes half-way through cooking time. Transfer to paper towel-lined plate and blot with additional paper towels. Serve.