Place the sweet potatoes into a large pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until tender, about 20 minutes. Drain and allow to steam dry for a minute or two.
Mash the potatoes until smooth, then stir in the hazelnuts, heavy cream, sour cream, and apple cider. Season to taste with salt and pepper.