Preheat your oven to 375°F (190°C). Grease the bottom and sides of your 8-inch springform pan and line the bottom with a circle of parchment paper.
In a heat-safe bowl, combine the chopped chocolate and butter. Melt them together using a double boiler or by microwaving in 30-second intervals, stirring between each, until completely smooth. Set aside to cool slightly.
Using an electric mixer, beat the eggs and sugar on medium-high speed for about 5 minutes. The mixture should become pale, thick, and triple in volume.
Gently fold the cooled chocolate mixture into the egg mixture using a rubber spatula. Be careful not to deflate the eggs; fold slowly until mostly combined.
Sift the cocoa powder and salt over the batter. Fold in gently until no streaks of dry powder remain.
Pour the batter into the prepared pan and smooth the top with your spatula.
Bake for 25 to 30 minutes. The cake is done when the top has formed a thin, crackly crust and the center is set but still has a slight jiggle.
Remove from the oven and let it cool in the pan on a wire rack for 15 minutes before releasing the springform ring. Serve at room temperature or slightly chilled.