Heat oven to 425°F. With a sharp knife, make vertical slits in potatoes, 1/8- to 1/4-inch apart, being sure to stop at least 1⁄4 inch before bottom (do not cut all the way through). Gently toss potatoes with oil and ½ teaspoon each flaky sea salt and pepper. Transfer to rimmed baking sheet, cut sides up. Roast until tender, golden brown, and crisp, 45 to 50 minutes. Meanwhile, in small bowl, combine butter and scallions. Dollop each potato with scallion butter.