Preheat oven to 350 degrees. Place washed and scrubbed sweet potatoes on a baking sheet. Bake potatoes for approximately 1 hour. The potatoes are done when a fork pierces through the potatoes. When potatoes have cool enough to handle, peel potatoes and place in a mixer. Add melted butter, horseradish sauce, and salt to the potatoes. Mix potatoes until smooth and creamy. Serve with additional melted butter if desired.