Preheat the fan oven to 170°C, 350°F, 4 Gas Mark. Grease the tin and cover it with baking paper.
Melt the chocolate and the butter in the bain-marie method*. Simmer at a lower heat until the mixture is melted. Stir often and set aside once done to cool it off.
Separate the egg yolk from the egg white. Add the egg yolk, vanilla seeds, and sugar in a stand mixture. Whip on medium speed until the mixture is thick and pale.
Whip the egg white in a stand mixer on medium speed until soft peaks.
Fold carefully, the melted chocolate mixture in the whipped yolks with a spatula. Once is well combined, start adding a spoon at the time the whipped egg whites and carefully fold from the bottom to the top.
Add the salt, and the cornstarch one spoon at a time in the batter and again fold gently until well combined.
Pour the batter into the tin, level up the mixture with the back of the spoon, and bake for 25 minutes*. The cake is ready when the top is dry and slightly cracking, and the toothpick comes slightly wet.
Once cooled off, remove it from the tin and dust the top with some cocoa powder.