Line your tin with baking paper.
Melt the chocolate.
In the bowl of a stand mixer, add the icing sugar, the vanilla and the butter.Beat until creamy. Separate the yolks and set the egg whites aside.
Add the yolks to the bowl.
Add the melted chocolate.
Meringue: in a separate bowl, whisk the egg whites with a pinch of salt and a tablespoon of lemon juice.
As soon as they start to hold, add the 100g of icing sugar in 3 additions.
Whisk until you have a meringue that is not too stiff. It should stay fairly loose.
Add a little meringue and a little flour to the chocolate mixture and fold gently with a spatula.Repeat the process 3 times. Pour the batter into the tin. The surface of the sponge should be fairly level.
Bake at 170°C for 15 minutes with the oven door slightly ajar.
Then 50 minutes at 170°C with the door closed.
Leave to cool, then carefully slice off the top of the sponge to remove the crusty layer.
Then cut the sponge into 2 or 3 layers of equal thickness.
The last layer, the one with a smooth side, goes on last, smooth side facing up.
Spread apricot jam over one layer.
Place the second layer on top. Do the same with the third.
Then coat the whole cake with jam, sides included.