Preheat the fan oven at 180°C, 350°F, 4 Gas Mark, grease tin, and covert it with baking paper.
Melt the chocolate and the butter in the bain-marie method. Simmer at lower heats until the mixture is melted. Set aside to cool off.
Separate the eggs yolk from the eggs white. Add the eggs yolk, vanilla seeds, and sugar in a stand mixture. Whip on a medium speed until the mixture is thick and pale.
Whip the eggs white in a stand mixer on medium speed until stiff 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 eggs whites and carefully fold from the bottom to the top.
Add the salt, and the cornflour, previously stiffed, one spoon at the 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 30 minutes. The cake is ready when pulling away from the tin, the top is starting to crack, and the toothpick comes clean but with crumbs.
Once cooled off, remove it from the tin and dust the top with some cocoa powder.