Preheat your oven to 350°F (175°C). Grease the bottom and sides of a 9-inch springform pan. Line the bottom with a circle of parchment paper, then grease the parchment as well. This prevents sticking and makes release easy.

In a heatproof bowl, combine the chopped dark chocolate and unsalted butter. Place the bowl over a saucepan of simmering water (ensure the bottom of the bowl does not touch the water). Stir occasionally until the chocolate and butter are completely melted and smooth. Alternatively, microwave in 30-second intervals, stirring after each, until melted. Remove from heat and let cool slightly for about 5-10 minutes.

In a large mixing bowl, whisk together the granulated sugar and eggs. If using an electric mixer, beat on medium-high speed for 3-5 minutes until the mixture is light in color, thick, and has increased in volume. If whisking by hand, aim for 5-7 minutes until noticeably thicker and lighter.

Slowly pour the slightly cooled chocolate and butter mixture into the egg and sugar mixture. Whisk gently but constantly until just combined. Be careful not to overmix, as this can incorporate too much air and affect the cake's dense texture.

Sift the unsweetened cocoa powder over the batter. Add the vanilla extract and salt. Using a rubber spatula, gently fold these ingredients into the batter until no dry streaks of cocoa powder remain. Again, avoid overmixing.

Pour the batter evenly into the prepared springform pan. Transfer the pan to the preheated oven and bake for 30-35 minutes. The cake is done when the edges are set and slightly puffed, but the center will still be slightly jiggly when gently shaken. A toothpick inserted into the center should come out with moist crumbs, not wet batter.

Remove the cake from the oven and place it on a wire rack to cool completely. The center of the cake will naturally sink as it cools, creating a signature dense texture. Once completely cool (this can take 1-2 hours), carefully run a thin knife around the edge of the pan before releasing the springform. Remove the parchment paper from the bottom.

To serve, dust with powdered sugar and garnish with fresh berries, if desired. Slice with a warm, dry knife for clean cuts.