Place potato cubes in a large pot and cover with cold water by 2 inches. Add 1 tablespoon salt and bring to a boil over high heat. Reduce to a simmer and cook until potatoes are just tender when pierced with a fork, 12 to 15 minutes. Drain immediately and spread on a baking sheet to cool completely, about 20 minutes. Warm potatoes will turn the dressing greasy.

While potatoes cool, melt butter with olive oil in a large skillet over medium-low heat. Add sliced onion and 1/2 teaspoon salt. Cook slowly, stirring every few minutes, until onions are deep golden brown and jammy, 20 to 25 minutes. If they brown too quickly, reduce heat. Set aside to cool to room temperature.

In a large mixing bowl, whisk together sour cream, mayonnaise, white wine vinegar, Dijon mustard, garlic powder, black pepper, and 1 teaspoon salt until smooth. Taste and adjust seasoning - it should be tangy and well-salted since potatoes will mute the flavor.

Add cooled potatoes and caramelized onions to the dressing. Fold gently with a rubber spatula until evenly coated, taking care not to break up the potatoes. Fold in most of the chives, reserving some for topping.

Cover and refrigerate at least 1 hour to let flavors meld. Taste and adjust salt before serving - cold dulls seasoning. Transfer to a serving bowl, top with remaining chives, and serve cold or slightly cool.