Heat oven to 450 degrees.
Toss potatoes, cornstarch, garlic powder, salt and pepper. Add oil to baking sheet, and place potatoes cut side down.
Roast until browned, about 30 minutes. Mix butter, garlic, parsley, and zest together in a bowl. Turn potatoes skin side down and roast until crisp 10-15 minutes.
Finish by tossing the potatoes with the butter and garlic mixture.