For the chocolate loops
Melt the chocolate couverture in a bain-marie and temper it. Cut the transfer sheets into 9 (12) strips (3-4 cm wide and 10-12 cm long). Spread the chocolate on the strips, allow to set slightly, then lift up the strips, straighten the edges with a sharp knife if necessary and shape into loops (with the transfer sheet to the outside). Place on foil and leave to set. Carefully pull off the transfer sheet before using the loops to decorate the cake.
For the sponge
Separate the eggs. Beat the egg whites with the lemon juice until they form soft peaks. Gradually trickle in the sugar and continue beating until the mixture is firm and shiny and forms stiff peaks. Whisk the egg yolks and fold in. Mix the flour and cocoa, sieve onto the mixture and fold in. Spread the mixture on a baking tray lined with baking parchment and bake in a preheated oven (200°C/400°F with top and bottom heat) for about 12 minutes. Take out of the oven, loosen the edge of the sponge and leave to go cold.
For the mousse
Roughly chop the chocolate and melt in a bain-marie with the cognac and espresso. Remove from the heat. Separate the eggs. Beat the egg yolks with the vanilla sugar over a hot bain-marie until foamy and the sugar has dissolved. Whip the well chilled cream until semi-stiff. Beat the egg whites with the sugar until they form stiff peaks. Add the egg yolk mixture to the chocolate and stir in with a whisk. Place the whipped cream on top of the chocolate mixture and quickly mix in with the whisk before the mixture sets. Carefully fold in the beaten egg white.
Split the cold sponge once and then split the two layers again, to make 4 thin layers of cake. Put a square cake ring around the bottom layer and spread with 1/4 of the chocolate mousse. Place the next layer on top and continue in this way until all the layers of cake and all the mousse have been used. Chill for at least 6 hours. (Freezing the cake for the last 15 minutes will make it easier to apply the icing).
For the icing
Put the sugar and water into a pan and boil vigorously for 5-6 minutes, then leave to cool slightly. Melt the chocolate in a bain-marie and gradually stir into the sugar solution. Stir until the icing is thick and smooth. Leave to cool until it feels pleasantly warm to the lips. Remove the cake ring. Pour the icing over the cake in one action and smooth quickly on all sides with a palette knife, using as few strokes as possible.
Top with the chocolate loops and leave to dry for several hours, until the icing is properly set. Cut into portions with a hot knife and serve.
To check that the icing has the right consistency, let it run over a wooden spoon. The spoon should be coated with a layer of icing approximately 4 mm/1/8 inch thick. Take care that the icing does not get too hot, or it will not be shiny.