Combine potatoes, 1 cup vinegar and 1 TB salt in a large saucepan over high heat.
Add water to cover potatoes by 1 inch.
Bring to a boil.
Cover and simmer until potatoes are tender, 15-20 min.
Preheat oven to 450 degrees F.
Line rimmed baking sheet with parchment or aluminum foil.
Whisk 4 cups cold water and remaining 4 TB salt in large bowl until salt is dissolved.
Add 2 cups ice.
Using slotted spoon, transfer potatoes to ice water.
Let stand 30 seconds, drain and pat dry.
Allow potatoes to cool for about 2 min.
Meanwhile, melt oil or butter in a small pan with pepper and 1 TB chives.
Remove from heat and whisk in remaining 2 tsp vinegar.
Using your heel of your hand, smash down each potato to about ½ inch thickness and lay on prepared baking sheet.
Brush potatoes with the oil/pepper/chive mixture and sprinkle with more salt.
Flip and repeat on the second side.
Bake about 30-40 minutes until crispy and golden.
Brush with more vinegar and serve immediately garnished with remaining chives and salt if desired.
