Peel and cut the potatoes into 2-inch chunks. Add to a pot of cold water with 1 tsp kosher salt, bring to a boil, then simmer for 18 to 20 minutes until fork-tender. Drain and rest in the colander for 2 minutes.

Warm the ½ cup heavy cream and 4 tbsp butter in a small saucepan over low heat for about 3 minutes until the butter melts and the cream is just warm.

Return the potatoes to the pot over low heat. Mash until no lumps remain, then fold in the warm cream and butter mixture. Add the ¼ cup sour cream, remaining ½ tsp kosher salt, and ½ tsp white pepper. Fold until smooth, taste, and adjust salt.

Spoon into a warm serving bowl, make a shallow well in the center, add the 1 tbsp softened butter, let it melt for 30 seconds, and scatter the 2 tbsp fresh chives over the top.