In a large saucepan, place the potatoes and cover with cold water. Cover the pan and bring to a boil over high heat. Reduce the heat to a simmer and cook for 25 to 30 minutes, or until a paring knife penetrates the potatoes easily; drain the potatoes in a colander. Mash the potatoes by working them through a food mill or by beating with a mixer. Transfer to a mixing bowl.
In a saucepan, melt the butter and, off the heat, whisk in the horseradish and sour cream. Fold into the potatoes with the chives and salmon; season with salt.