Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes. Drain the potato water and swivel the potatoes dry on the still hot cooking place. Press through a potato ricer.
Stir in 3 egg yolks, butter and 5 tablespoons of milk and season to taste with salt and nutmeg. Pour into a piping bag with a large star-shaped spout and squirt about 18 duchess potatoes onto the tray. Whisk 1 egg yolk with 2 tablespoons of milk and carefully brush the potatoes with it.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 15 minutes.