Heat oven to 425 degrees F. Make slits in the potatoes, making sure to stop at least 1⁄4 in. before the bottom (do not cut all the way through).
Gently toss the potatoes with the oil and 1⁄2 tsp each flaked salt and pepper. Transfer to a large rimmed baking sheet, cut-sides up. Roast until tender, golden brown and crisp, 55 to 60 minutes.
Meanwhile, in a small bowl, combine the butter and chives. Dollop small spoonfuls of the chive butter over the potatoes, gently toss, then transfer to a serving platter.