Shred the potatoes to spaghetti through the medium blade of a mouli-julienne or food processor. Rinse in a basin of water, drain and roll tightly in a kitchen towel to dry.
Melt half the butter in a heavy frying pan over low to medium heat. Pack in the potatoes, smoothing the surface and edges carefully with the back of a fork, and season the surface. Place strips of butter all around t