Preheat the oven to 325 F.
Grease the inside of a 9-inch springform pan. Line the bottom with parchment paper.
Fill a medium pot half full of water. Bring it to a simmer.
Meanwhile, chop the chocolate into matchstick-sized pieces.
Place the chocolate in a large stainless-steel bowl. Add the butter.
Place the bowl over the simmering water. Stir occasionally, until the contents are melted.
In a medium bowl, sift the cocoa powder. Stir in the sugar.
Add to the chocolate mixture, whisking well until dissolved. Remove from heat to cool.
Once cool, whisk in the egg yolks until combined.
In a medium bowl, whip the eggs to stiff peaks.
Fold the egg whites into the chocolate mixture using one-third at a time.
Pour the cake batter into the prepared pan. Bake for 40-45 minutes until set.