Preheat oven to 450°F. Set a rimmed baking sheet inside to heat up.
Bring a large pot of water to a boil. Stir in baking soda and 1 teaspoon salt.
Add potatoes and simmer 8–10 minutes until the edges look soft.
Drain well and let steam off for 2 minutes. Shake the colander to rough up the surfaces.
Toss potatoes with cornstarch, garlic powder, paprika, remaining salt, and pepper until coated.
Drizzle with olive oil and sprinkle in rosemary. Toss to evenly coat.
Spread potatoes on the hot baking sheet in a single layer, cut sides down when possible.
Roast 20 minutes, flip, then roast 15–20 minutes more until deep golden and crisp.
Shower with parsley and season to taste. Serve immediately.