Preheat the oven to 350°F (180°C). Line a 9‑inch (23 cm) round cake pan with parchment paper on the bottom and lightly grease the sides.
Melt the dark chocolate and butter together over a bain‑marie (heatproof bowl set over a pot of gently simmering water), stirring until completely smooth and glossy. Take your time so the chocolate doesn’t overheat or seize.
Remove the bowl from the heat and let the chocolate mixture cool slightly. Add the 8 egg yolks one by one, whisking well after each addition until you have a thick, shiny chocolate base.
Add the sifted cocoa powder and salt to the chocolate mixture and whisk again until everything is evenly combined with no visible dry spots.
In a separate clean bowl, whip the 8 egg whites with the sugar until you get stiff, glossy peaks; this aeration is what gives the cake its light, mousse‑like texture without any flour.
Gently fold about 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, using a spatula and broad, gentle movements so you don’t deflate the batter.
Pour the batter into the prepared pan, smooth the top and bake at 350°F (180°C) for about 30 minutes, until the edges are set and the center is just slightly soft and puffed.
Let the cake cool in the pan for 5–10 minutes, then remove the ring or carefully lift it out onto a rack and leave it to cool completely at room temperature. The cake will sink slightly as it cools – this is normal for a flourless chocolate cake and gives it a dense, fudgy texture.
Once completely cool, dust the top of the cake with cocoa powder and powdered sugar to taste. Store any leftovers covered in the refrigerator for up to 5–6 days, bringing slices to room temperature before serving for the best texture.