Combine potatoes, 1 cup vinegar, and 1 Tbsp. kosher salt in a medium saucepan; add water to cover by 1”. Bring to a boil, reduce heat, and simmer until potatoes are just tender, about 20 minutes.
Line a baking sheet with parchment paper.
After the potatoes have boiled for 10 minutes, preheat oven to 450°.
When potatoes are tender, drain and return to saucepan. Add butter and gently toss the potatoes to coat. Transfer potatoes to lined baking sheet, spreading them out in a single layer. Use a heavy mug or glass to smash each potato to about ½-inch thickness.
Bake for 20 minutes.
Remove potatoes from oven and turn with a spatula. Drizzle with olive oil and return to oven for 20 minutes more.
Remove from oven and sprinkle with 2 tablespoons vinegar. Top with chives, and black pepper and sea salt to taste. Serve.