Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes. Drain, and mash with 1 tablespoon butter and black pepper. Stir in sour cream, horseradish and parsley. Whip potatoes and place in medium serving bowl.
Melt remaining 1 tablespoon butter and pour over potatoes. Serve immediately.