Preheat the oven to 350 degrees . Cut a small sliver off each potato so that it will stand flat when filled. Place the potatoes in a medium pot, add enough cold water to cover by 1 inch and add a few generous pinches of salt. Bring to a boil and cook until fork-tender, 10 to 12 minutes. Drain and let the potatoes sit until cool enough to handle.
Using a melon baller or measuring spoon, hollow out each potato, reserving the scooped flesh in a medium bowl. Add the sour cream and horseradish to the bowl and smash with a potato masher (or run through a ricer); season with salt and white pepper.
Spoon the mashed potato mixture back into the hollowed-out potatoes (save any leftover filling for another use). Bake on a foil-lined baking sheet until heated through, about 15 minutes.