Preheat the oven to 450F. Line a large rimmed baking sheet with heavy-duty foil and lightly coat with cooking spray or olive oil.
Place the potatoes in a large pot and cover with cold, well-salted water by 1 inch. Bring to a boil and cook until just fork tender, 15-20 minutes. Drain well and let steam dry for 5 minutes.
Arrange the potatoes on the baking sheet with about 2 inches between each. Using the bottom of a sturdy glass or measuring cup, gently smash each potato to about 1/2 inch thick.
In a small bowl, combine the olive oil, 1/2 teaspoon of the kosher salt, garlic powder, onion powder, black pepper, and smoked paprika. Brush or spoon evenly over each potato.
Roast until the bottoms are deeply golden and the edges are crisp, 20-25 minutes.
Sprinkle the shredded cheddar evenly over the potatoes and return to the oven until the cheese is fully melted and just beginning to brown, 5-7 minutes.
Finish with the remaining 1/4 teaspoon kosher salt, flaky sea salt if using, and chives or parsley. Serve immediately.
