Heat oven to 180C/fan 160C/gas 4. Prick potatoes with a fork. Place on a baking sheet, then bake for 1 hr until skins are very crisp and the potato flesh is tender.
Turn oven to 200C/fan 180C/gas 6. Cut potatoes in half, scoop out flesh. Cut potato shells into quarters, brush with melted butter. Return to oven for 10-20 mins until crispy. Sprinkle with sea salt and black pepper to serve.