
Fill a large pot with cold, salted water. Bring to a boil.
Add the potatoes and cook until tender-yet-firm, about 15 minutes. Do not overcook!
Drain the potatoes and return to the hot pot. Add butter and blend with a hand mixer until butter is mixed in, but the potatoes are still chunky.
Blend in the sour cream and horseradish until smooth and whipped. Mix in salt and pepper to taste. Serve immediately.