Heat oven to 425°F (218°C). Set a wire rack on a rimmed sheet (or bake directly on the oven rack).
Scrub potatoes. Dry completely. Prick each 6–8 times with a fork.
Rub a thin coat of oil on the skins (optional). Sprinkle all sides with kosher salt.
Bake on the rack with space between until centers reach 205–212°F (96–100°C) or a fork slides in easily, 45–65 minutes.
Rest 5 minutes. Split lengthwise (or make an X). Squeeze ends to fluff. Top each with 1 tbsp butter, chives, and black pepper. Serve hot.