Preheat the oven to 180 degrees Celsius. Prepare a 23 cm diameter baking pan and line it with baking paper.
Melt the chocolate and butter in a double boiler. Take your time, the melted chocolate should be smooth and uniform.
Allow the chocolate to cool slightly, then add 8 egg yolks. Mix well until smooth.
Add the cocoa powder and salt to the chocolate mixture. Mix well again to ensure that all ingredients are evenly distributed.
Beat the egg whites with the sugar until stiff peaks form. This is important for the cake to be fluffy and fine.
Add 1/3 of the beaten egg whites to the chocolate mixture and mix gently. Then add the remaining beaten egg whites and mix again, making sure the consistency remains fluffy.
Pour the batter into the pan, flatten it, and bake at 180°C (350°F) for 30 minutes.
After baking, let the cake cool for 5-10 minutes in the pan. Then remove the pan and leave the cake at room temperature until it cools completely. Important: the flourless cake will settle slightly after cooling. This is normal!
Sprinkle the cake with cocoa powder and powdered sugar to taste. The chocolate cake can be stored in the refrigerator for 5-6 days.