Bring milk, 1/8 litre water, salt and butter to the boil in a saucepan. Add the flour at once and stir with a wooden spoon until the dough separates as a lump from the bottom of the pot. Stir in 1 egg, allow to cool for 10 minutes, stir in 3 eggs one after the other.
Put the choux pastry into a piping bag with a large star-shaped spout and squirt 12 cream puffs (3 cm wide each) onto a baking tray (approx. 32 x 37 cm) lined with baking paper. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 25 minutes.
Meanwhile, chop the chocolate coating roughly and melt it over a warm water bath. Let it cool down a little. Separate 2 eggs. Beat the egg yolks, sugar and vanilla sugar in a tall beaker with the whisks of the hand mixer until creamy.
Separately beat egg whites and cream until stiff. Let the chocolate coating slowly run into the egg yolk while stirring. Stir in half of the cream immediately with a whisk. Fold in the remaining cream and then fold in the beaten egg white in portions.
Chill for 1/2 hour. Cut the cream puff open horizontally. Pour chocolate mousse into a piping bag with a large star-shaped spout and spray onto the bases. Place the lid on top and sprinkle with chocolate shavings.