Add potatoes, white vinegar, and salt into a large pot. Cover with water by 1 inch, and bring to a boil.
Once the potatoes are tender (about 15 minutes), transfer to a cast iron pan and coat with butter. Using a potato masher or the bottom of a heavy container, smash the potatoes into the bottom of the pan until flattened.
Bake the potatoes in a 500-degree oven for 30 minutes, flipping halfway through. Turn the oven to a high broil, and broil for 5 more minutes.
Brush the potatoes with the malt vinegar, and top with chopped chives. Serve warm.