Preheat oven to 400°. Rub potatoes with oil. Place them on a baking sheet and sprinkle each with a pinch of salt. Bake until tender, 45–60 minutes. Transfer potatoes to a cutting board and let sit until cool enough to handle.

Cut a ½-inch-thick slice off the top of each potato; scrape the flesh from the tops into a large bowl. Spoon the flesh from the potatoes into the bowl, leaving a ¼-inch shell. Add butter, ½ teaspoon salt, and pepper. Mix with a potato masher until mostly blended (lumps are OK). Gently fold in the crème fraîche and half the cheese.

Using your fingers, rub the outsides of the potato shells with the oil. Distribute the filling among the shells, mounding it, and place on a rimmed baking sheet.

Bake the potatoes, uncovered, until heated through, 25–30 minutes. (To check, insert the blade of a small knife into the center of a potato for 5 seconds; it should be hot.) Sprinkle the potatoes with the remaining cheese and bake until the cheese melts and browns lightly in some places, 2–3 minutes more. Garnish the potatoes with the chives, let sit 5–10 minutes, and serve.