In saucepan, bring water and 1 teaspoon salt to boil. Add potatoes and boil 15-20 minutes. Reserve 1/2 cup liquid.
Drain potatoes and place in bowl. Add six slices of butter and mash potatoes coarsely.
In medium bowl, whisk together sour cream, horseradish, remaining salt, pepper and chives. Add mixture to mashed potatoes. Add reserved water. Mash to combine.
Transfer to serving bowl and top with remaining butter pieces and additional chives.