Preheat oven to 450 degrees F. Line a large baking sheet with parchment paper.
Add potatoes and 1 Tbsp. kosher salt to a medium saucepan. Cover with water and bring to a boil. Reduce heat and simmer until potatoes are tender, about 15 minutes.
Drain and return potatoes to saucepan. Add butter and gently toss to coat. Transfer potatoes to prepared baking sheet, spreading them out in a single layer. Using a heavy mug or glass, smash each potato to about 1/2-inch thickness.
Bake for 20 minutes. Remove potatoes from oven and turn each with a spatula. Drizzle with olive oil and continue baking for 20 minutes more.
Once baked, sprinkle with vinegar, chopped chives, salt and pepper. Serve hot.