Trim all the rounded edges off the potatoes to make a rectangular block, then cut this into thick batons (save any offcuts to make mash). Drop into a pan of cold salted water with the vinegar and bring to the boil. Simmer for 6-8 mins until the potatoes are just cooked and tender – test with the tip of a knife. Drain carefully, trying not to break any of the chips, and leave to cool completely. Can be prepared up to two days ahead and chilled until needed.
Heat the oven to 220C/200C fan/gas 9. Pour the oil into a sturdy, rimmed baking tray or shallow roasting tin and heat in the oven. Mix the flours with the baking powder, cayenne and some salt in a shallow dish and gently toss the chips in the floury mixture to coat, then leave in the mixture until needed.
Carefully remove the pan from the oven – the oil should be shimmering – and lay the chips in a single layer in the pan. Use a thin, flexible spatula to gently turn the chips so they are all coated, then roast for 20 mins. Turn the chips again, then roast for another 10 minutes. Turn one final time, then cook for a futher 10 minutes until crisp and deep golden all over. Drain the chips onto kitchen paper and serve straight away.