Preheat your oven to 375°F (190°C) and grease an 8-inch round springform pan with butter or non-stick spray. Line the bottom with parchment paper to ensure the delicate cake releases easily after baking.

In a heatproof bowl set over a pot of simmering water, melt the chopped dark chocolate and butter together, stirring occasionally until smooth and glossy. Remove from heat and let the mixture cool slightly for about 5 minutes.

In a large mixing bowl or stand mixer, whisk the eggs, granulated sugar, vanilla extract, and salt on medium-high speed until the mixture is pale and thick, about 4 to 5 minutes.

Gently fold the cooled chocolate mixture into the egg mixture using a spatula, being careful not to deflate the eggs too much, until just combined.

Sift the cocoa powder over the batter and gently fold it in until no dry streaks remain and the batter is uniform in color.

Pour the batter into the prepared springform pan and smooth the top with your spatula.

Bake in the preheated oven for 25 to 30 minutes, until the top has formed a thin crust and the center registers 200°F (93°C) on an instant-read thermometer, though it may still look slightly wobbly.

Remove the cake from the oven and place it on a wire rack to cool completely in the pan; the cake will deflate slightly and crack as it cools, which is characteristic of this style.

Once cool, run a knife around the edge of the pan to loosen the cake, then release the springform ring.

Dust the top generously with powdered sugar before slicing and serving with fresh berries or a dollop of whipped cream.