Scrub the potatoes, dry them well, and pierce them all over with the tines of a fork. Place the potatoes on the prepared sheet pan and rub them all over with olive oil. Roll each potato in the salt and herb mixture and put them back on the sheet pan. Bake for 60 to 75 minutes, until tender. Slit the top of each potato, squeeze the ends together to open the potato, and top with a generous scoop of whipped feta. Sprinkle with chives and serve hot with extra whipped feta on the side.