Separate the egg yolks from the whites and beat the whites until stiff peaks form.

Bring water to a boil in a saucepan, filling it halfway. Place a bowl on top to melt the bittersweet chocolate. Let it cool slightly.

In a bowl, combine all the dry ingredients and mix well.

In another bowl, beat the softened butter, powdered sugar, and vanilla with a mixer until very soft and creamy.

Pour the melted chocolate into the butter mixture, mix, then add the egg yolks and mix well. Add the dry ingredients and mix with a hand whisk. Gradually fold in the egg whites a little at a time. The batter should now be thick and just right.

Brush a 9-inch (24 cm) cake pan with baking spray or line with parchment paper. Pour in the batter and smooth the top. Bake at 340°F (170°C) for 30 minutes, then turn off the oven and leave the cake inside for another 12–15 minutes. Remove from the oven, let cool in the pan, then take the cake out and let it rest on a wire rack for 5–6 hours. Dust with powdered sugar and, if you like, decorate with cocoa powder.

Tip: This delicate cake can be tricky, so follow each step carefully!
* Eggs should always be at room temperature! If you forget, leave them in warm water for 3–4 minutes. Do the same for the butter.
* Use high-quality ingredients.
* When folding in the egg whites, always mix from the bottom up to avoid deflating them.

Always do the toothpick test! That’s how you’ll know if the cake is done—it should come out moist with a few crumbs.
* Let the cake cool before removing it from the pan to avoid breaking it.
* Cool the cake on a wire rack so it stays tall and doesn’t get soggy.
* Only dust with powdered sugar when the cake is completely cool, or it will melt.