Place diced potatoes in a large saucier or pot and cover with water. Season generously with salt. Place potato skins in a fine-mesh strainer and place on top of pot. Add just enough water to submerge potato skins. Bring to a boil over high heat and simmer until potatoes are tender, about 15 minutes.
Discard potato skins, drain potatoes, and transfer to a rimmed baking sheet. Immediately sprinkle with 2 tbsp of vinegar and set aside to cool. When they are cool enough to handle, transfer potatoes to a large bowl.
Add remaining vinegar, olive oil, mustard, red onion, chives, chicken stock, and sugar. Roughly stir and fold mixture so that potatoes release some starch and liquid begins to thicken a little. Season to taste with more salt and black pepper. Set aside to rest for at least 30 minutes and up to overnight. Stir again vigorously to thicken dressing; it should have a loose but not soupy consistency. If it's too thick, thin it out with a little extra water or chicken stock and re-season. Serve cold or at room temperature.
Stir again vigorously to thicken dressing; it should have a loose but not soupy consistency. If it's too thick, thin it out with a little extra water or chicken stock and re-season. Serve cold or at room temperature.
Enjoy!
