Peel the potatoes and boil them in a pot of water. Cook for 20-30 minutes or until tender. Drain and set aside in the same cooking pot.
Place the butter in a pan and let it melt. Add the milk and mix well. Pour this mixture over the potatoes and crush them with a potato masher. Salt to taste and add a pinch of pepper.
Serve in individual glasses and add a tablespoon of black Mújol Shikrán on top. Decorate with a fresh rosemary twig.