In a medium saucepan, combine together whole potatoes, cup of vinegar, and 1 tablespoon salt. Add water to cover the potatoes by 2". Bring the water to a boil, then reduce the heat down to simmer, cooking potatoes for about 20 minutes.
About 10 minutes into boiling the potatoes, pre-heat the oven to 450 degrees. Cover a baking sheet with parchment paper.
Continue cooking potatoes for another 10 minutes. Poke them with a fork to check for doneness, they should be just tender.
Drain potatoes and return to pan to dry. Coat the potatoes with the butter. Lay potatoes out on the lined baking sheet. Using a heavy mug, gently smash each potato to 1/2" thick.
Bake the potatoes for 20 minutes.
Turn each potato over and drizzle with olive oil. Return to oven for an additional 20 minutes.
Broil potatoes for a minute or two on high to get extra crispy. Be sure to pay close attention so they do not burn.
Take out of oven and top with 2 tablespoons of vinegar, chives (if using), sea salt, and pepper.