Preheat oven to 325°F (160°C). Grease an 8-inch (20cm) springform pan, line the bottom with parchment paper, and grease the parchment. Set aside.

In a large heatproof bowl set over a saucepan of simmering water (bain-marie), melt the chopped dark chocolate and butter, stirring occasionally until smooth. Remove from heat and let cool slightly.

In a separate medium bowl, whisk together the egg yolks, 1/2 cup (100g) of the granulated sugar, sifted cocoa powder, vanilla extract, and salt until smooth and well combined.

Gradually whisk the slightly cooled chocolate mixture into the egg yolk mixture until fully incorporated and smooth.

In a very clean, dry bowl (preferably metal or glass) using an electric mixer, beat the egg whites on medium speed until soft peaks form. Gradually add the remaining 1/2 cup (100g) granulated sugar, increasing speed to high, and continue beating until stiff, glossy peaks form. Be careful not to overbeat.

Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten it. Then, carefully fold in the remaining egg whites in two additions, taking care not to deflate the mixture. Mix just until no white streaks remain.

Pour the batter into the prepared springform pan. Bake for 30-35 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs (not wet batter). The center may still be slightly jiggly, which is good for a fudgy texture.

Remove the cake from the oven and let it cool completely in the pan on a wire rack. As it cools, the center will slightly sink and crack, which is normal for a flourless cake. Once cool, run a knife around the edges before releasing from the springform pan.

While the cake cools, prepare the ganache if using. Heat heavy cream in a small saucepan over medium heat until it just begins to simmer. Remove from heat and pour over the chopped dark chocolate in a heatproof bowl. Let sit for 5 minutes, then whisk until smooth and glossy.

Pour the slightly cooled ganache over the completely cooled cake, spreading evenly. Let the ganache set slightly at room temperature or in the refrigerator for 15-30 minutes before slicing and serving.