Place potato in a large saucepan of cold salted water. Bring to the boil and cook for 10 minutes, or until tender. Drain, transfer to a large bowl and roughly mash.
Heat 1 tablespoon of oil in a large non-stick frypan over high heat. Sprinkle salmon with salt and pepper and cook for 4 minutes on each side.
Flake the salmon. Add salmon, parsley, sour cream, horseradish, egg, salt, pepper and 1/2 cup of the breadcrumbs to the potato and mix until combined. Shape 1/3 cup of the mixture into a patty. Repeat with the remaining mixture to make 8 patties.
Place the remaining breadcrumbs on a tray and roll the patties in the crumbs to coat.
Heat the remaining oil in a large non-stick frypan over high heat. Cook the salmon and horseradish cakes in batches for 3 minutes each side. Sprinkle with salt. Mix the extra sour cream and extra horseradish in a small bowl and serve with the cakes.