Bake the potatoes until a toothpick easily penetrate them.
Cut each potatoes in half and remove the flesh leaving the skin intact.
Process the flesh through a potato ricer.
Add butter, sour cream, horseradish, salt, and pepper to the riced potatoes. Mix well.
Whip egg whites to soft peaks. Fold in to potatoes mix.
Mound the potato mix into the skins.
Cover with shredded cheese. Bake at 350 degrees F until the cheese begins to brown.
