Preheat your oven to 325°F (165°C) and grease a 9-inch round cake pan, then line the bottom with parchment paper.
In a heatproof bowl set over a saucepan of simmering water, melt the chopped bittersweet chocolate and butter, stirring occasionally until smooth and well combined.
Remove the bowl from the heat and whisk in the granulated sugar until it dissolves completely.
Add the eggs, one at a time, whisking well after each addition.
Sift the cocoa powder into the mixture and add the vanilla extract and salt. Gently fold everything together until the batter is smooth and glossy.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake in the preheated oven for approximately 35-40 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack or refrigerate to cool completely.