Pre-heat oven to 450 degrees. Rinse and quarter the potatoes. (Make sure the pieces are fairly uniform, about 1 inch.)
Place the potatoes in a large sipper-top bag with the cornstarch, rosemary, garlic powder, salt and pepper. Shake well to coat. Transfer the potatoes to a large baking pan and spread them in a single layer.
Drizzle oil over the potatoes and bake for 30 minutes, stirring once halfway through. Serve immediately.
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