Heat oven to 160C/140C fan/gas 3. Put the egg whites and salt in 2 shallow bowls. Dip each potato into the egg white to completely cover, then dab into the salt to cover in patches. Carefully put the potatoes onto a baking sheet, pressing on any remaining salt, then bake for 1½ hrs.
Bake them for a further 1 hr at 200C/180C fan/ gas 6. Scrape off as little or as much of the salt as you like before eating.