Sweat mushrooms in hot butter and season. Chop parsley, fold in. Press cooked potatoes through a potato press and mix them while still warm with flour, 35 g butter and 1 egg yolk.
Season the dough, fold in the herbs. Roll out the dough on a floured work surface and cut out circles, triangles etc. Spread mushrooms on half of the cut out dough pieces. Cover with remaining dough pieces.
Whisk the rest of the egg yolk and brush the edges with it, press down with a fork. Put the Maultaschen in boiling salted water and let them steep for about 12 minutes. In the meantime clean, wash and cut leek into rings.
Cook in 200 ml boiling salted water for 8 minutes. Drain and collect the stock. Swivel leek in hot butter. Deglaze with stock and cream, bring to the boil and thicken with sauce thickener. Season with salt and nutmeg.
Fry Maultaschen in remaining butter with parsley leaves, serve.