Place the Young Potatoes in a medium pot, cover with Water and add a pinch of Salt. Bring to a boil, then simmer until the potatoes are just tender, about 12–15 minutes depending on size. Drain, return to the pot off the heat and toss with the Butter until melted; let cool, then cut into bite-sized pieces.
While the potatoes cook, hard-boil the Eggs: place them in a small saucepan, cover with cold water, bring to a gentle boil and cook 8–10 minutes. Transfer to ice water, peel and chop when cool.
In a large bowl whisk together the Kefir and the Sour Cream until smooth. If you prefer a thinner soup, whisk in a little more cold water to reach desired consistency.
Prepare the vegetables and herbs: grate or finely dice the Cucumber (peel if the skin is thick) and thinly slice the Radishes. Finely chop the Dill and the Chives.
Add the cucumber, radishes, dill and chives to the kefir mixture. Season with Lemon Juice, a small pinch of Sugar, Fine Salt to taste and freshly ground Black Pepper. Taste and adjust acidity, salt or sugar as needed — the soup should be bright and slightly tangy.
Cover the bowl and chill the soup thoroughly in the refrigerator for at least 1 hour to let the flavors meld and the soup become cold.
To serve, ladle the chilled chłodnik into bowls and top with the chopped hard-boiled eggs and the buttered young potatoes. Garnish with a little extra chopped Dill or Chives if desired.
Store leftovers refrigerated up to 2 days; stir before serving as ingredients may settle.
