Peel the potatoes, cut into pieces and boil in salted water. Cut the leek into 1 cm thick rings. Lightly brown 50 g butter and sauté the leek in it.
Roast pine nuts without adding any fat. Heat the milk with 30 g butter.
Drain the potatoes and chop them up with the masher. Gradually stir in the hot milk with the whisk. Fold the leek rings under the potatoes and sprinkle with the pine nuts.