For the Halloween ghost cake, first preheat the oven to 180 °C top and bottom heat. Butter the bottom of the springform pan.
Separate the eggs and whisk the egg yolks with half of the sugar and the vanilla sugar until foamy.
Beat the egg whites with the salt until stiff, sift in the remaining sugar and beat until very stiff. Fold the beaten egg whites into the egg yolk mixture with a mixing spoon.
Mix the flour with the almonds, cornstarch and cocoa and fold into the mixture.
Pour the batter into the prepared springform pan, smooth it out and bake on the middle shelf in the preheated oven for about 40 minutes (test with a stick). Remove the finished base from the oven, let it cool slightly, carefully loosen it from the edge of the springform pan with a thin knife, turn it out onto a kitchen rack and let it cool completely for at least 2 hours.
For the mousse, coarsely chop the couverture and melt it together with the cognac and espresso over a hot water bath. Remove from the heat and allow to cool.
Separate the eggs and beat the egg yolks with the vanilla sugar until foamy.
Whip the well chilled whipped cream until stiff.
Beat the egg whites with the sugar until stiff peaks form.
Add the egg yolk mixture to the chocolate, sift the cocoa over it and stir in with a whisk. Place the whipped cream on top of the chocolate mixture and fold in with a whisk, then carefully fold in the beaten egg whites.
Divide the sponge horizontally into 3 layers of about the same thickness.