Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes. Drain the potatoes and swivel them dry on the still hot stove. Press them through a potato press while still hot.
Stir in 3 egg yolks and butter. Season to taste with salt and nutmeg. Pour into a piping bag with a large star-shaped spout and squirt about 30 duchess potatoes onto a baking tray lined with baking paper.
Whisk 1 egg yolk. Gently brush the potatoes with it. Bake the potatoes in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 15 minutes.