Place peeled, cut potatoes in a 6–8 quart pot and cover with cold water by 1 inch; bring to a boil over high heat, reduce to a gentle simmer and cook 10–12 minutes until a fork slides in with slight resistance.

Drain the potatoes in a colander, return them to the warm pot off the heat, cover, and let steam for 2–3 minutes; transfer to a large heatproof bowl and allow to cool slightly.

Place eggs in a small pot, cover with cold water, bring to a boil, then cover and remove from heat; let sit 10 minutes, then transfer to an ice bath and cool completely before peeling and chopping.

In a medium bowl whisk together mayonnaise, buttermilk, yellow mustard, apple cider vinegar, sugar, salt, and pepper until combined; fold in chopped dill pickles and optional sweet relish and celery.

With the potatoes warm (not hot), add half the dressing and gently fold to coat; add chopped eggs and fold again, then add more dressing to reach the desired scoopable consistency—avoid oversaturating.

Refrigerate at least 1 hour before serving to let flavors meld; before serving, taste and adjust salt or a small splash of vinegar, then garnish with sliced scallions and a sprinkle of paprika.