Heat oven to 220C/200C fan/gas 8. Cut a circle of baking parchment to fit the base of a 20-23cm ovenproof frying pan. Using a mandolin or sharp knife, slice the potatoes as thinly as possible. Pour a little of the butter over the parchment on the base and neatly overlap slices of the potato on the first layer. Then pile on one-third of the remaining potatoes, season, drizzle with a little butter and repeat until all the potatoes are used.
Put the pork skin, skin-side up, on top of the potatoes and roast everything for 1 hr 30 mins until the crackling is crispy and the potatoes are really crisp and golden. Set the crackling aside, turn the potatoes out onto a tray or board and serve the crackling on top.