Peel, wash and halve the potatoes and boil them in salted water for about 20 minutes. Drain potatoes, add 100 ml milk and 1 egg yolk and mash finely. Season to taste with salt, pepper and nutmeg. Pour into a large piping bag with a large star-shaped spout.
Line a baking tray with baking paper. Spray 12 duchess potatoes on it. Whisk 1 egg yolk with 2 tablespoons of milk and carefully brush the duchess potatoes with it. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 8 minutes