Put the potatoes in a saucepan. Pour 1 cup vinegar over them, and add the salt.
Top up with enough cold water to just cover the potatoes.
Bring to a boil and cook for about 7 minutes, or until you can just insert a fork into them but they aren’t yet cooked through.
Drain immediately and let sit in the colander for a few minutes to steam dry.
Return the potatoes to the saucepan you boiled them in and toss with the olive oil; then spread out on a large baking sheet in a single layer with room between each potato.
Bake for 30 minutes, and then remove from the oven.
Smash each potato with a potato masher or the bottom of a mug so it flattens and splits.
Drizzle with the remaining ¼ cup (60 ml) of vinegar, making sure to do it as evenly as you can. Then return the pan to the oven and bake again for about another 30 minutes, or until really golden and crispy.
Sprinkle with a generous scattering of flaky sea salt and serve immediately