1/ Prepare the cake pan: Grease the cake pan with butter and line the bottom with parchment paper. Lining the sides with paper is also recommended for easy removal.
2/ Melt the chocolate and butter: Place the bittersweet chocolate and unsalted butter in a large bowl and set it over a pot of simmering water (double boiler). Melt gently, stirring occasionally, until completely smooth and glossy.

3/ Beat the egg yolks: Separate the eggs into yolks and whites. Add half of the sugar (50 g) and the vanilla sugar (or a few drops of vanilla extract) to the yolks and whisk until thick and pale.
4/ Make the base batter: Check that the melted chocolate mixture is not too hot, then add it to the egg yolk mixture. Sift in the cocoa powder and mix until fully combined.

5/
Make the meringue: In a separate bowl, add a pinch of salt to the egg whites and start beating. Gradually add the remaining 50 g of sugar in 2–3 additions while continuing to beat until stiff peaks form.
6/
Fold the meringue into the batter: Add the meringue to the chocolate mixture in three portions, folding gently with a spatula each time. Continue folding until no white streaks remain and the batter is smooth and airy.

7/
Prepare the water bath: Pour hot water into a baking tray to create a water bath for baking.
8/
Pour the batter into the pan: Pour the batter into the prepared pan and smooth the surface lightly.
9/ Bake in a water bath: Place the cake pan in the water bath and bake at 170°C/340°F(fan), or 190°C/375°F(conventional) for 30–35 minutes.
10/ Cool gradually in the oven: Once baked, turn off the oven and leave the door slightly open. Let the cake rest inside for 10–15 minutes to cool gradually.
11/
Cool completely in the pan: Remove the cake from the oven and let it cool in the pan. Once completely cool, remove the side of the pan and gently invert the cake onto a plate to release it cleanly.
12/
Chill in the fridge: Refrigerate for 1–2 hours, or overnight for best texture.
13/
Finish and serve: Dust the top with cocoa powder or powdered sugar before serving.

