Preheat oven at 180°C/350°F. Line the bottom of a 22cm (9-inch) springform pan with parchment paper.
Melt the chocolate on the top of a double boiler over simmering water (the bottom of the pan should not touch the water). Remove from the heat, add butter and stir until melted.
In a large bowl, beat 2 whole eggs and 4 yolks. Add ½ cup of the sugar and beat. Add chocolate mixture and beat.
In another bowl, whip the remaining 4 egg whites with a pinch of salt until foamy. Gradually add the remaining ½ cup sugar and beat until stiff peaks form. Gently fold them into the chocolate mixture.
Pour the batter into the prepared pan and bake until the top is flaky and the center is almost set, 35-40 minutes. It is very important not to overcook. Let it cool at room temperature. Do not worry if the cake sinks in the middle, it is perfectly fine! Whip cream and powdered sugar, spread it over the top of the cake and decorate with chocolate shavings or cocoa powder.
