Clean, wash and cut the leek. Cook until soft in boiling salted water and then rinse in cold water. Squeeze the leek well and puree with the liquid butter in a mixer.
Peel the potatoes, cut them into small pieces and cook them in salted water until soft. Drain and press through a press while it is still hot.
Bring the milk to the boil and pour over the potatoes. Stir in the wasabi paste (careful - very spicy) and the leek puree. Finally, season with salt.