Preheat oven to 350°F (177°C). Grease and line the interior of an 8 inch round springform cake pan with parchment paper.
Separate the egg whites and yolks for four of the eggs. Add egg whites to a clean mixing bowl and set aside.
Add egg yolks, remaining 2 eggs and sugar to a small mixing bowl. Whisk until eggs are completely beaten and egg whites are starting to get a little frothy (about 30 seconds).
In a separate large mixing bowl, add chocolate and butter. Melt chocolate and butter in the microwave in 15-30 second intervals, stirring in between with a whisk, until chocolate mixture is completely melted, smooth and uniform in color. You can also do this on the stove using the double boiler method.
Add egg yolk mixture to the chocolate bowl. Mix with a whisk until the batter is smooth and uniform.
Beat egg whites at highest speed until stiff peaks form. I used a stand mixer but you can also use a hand mixer.
Gently fold in the egg whites into the chocolate batter. Do this in four batches. Add 1/4 of the egg white mixture to your chocolate batter. Fold it in gently with a spatula until no egg white lumps remain (it's okay if there are a few streaks of white in your batter). Make sure you scoop up batter from the very bottom of the bowl each time you fold to make sure all of the batter gets incorporated with the egg whites. Repeat with the next 1/4, then the next 1/4 and then final 1/4.
Pour batter into prepared baking pan. Place cake into the oven and bake for about 40 minutes. When cake is done, the top should look puffed and slightly cracked. A toothpick inserted should come out clean.
As the cake cools, the top will collapse/sink in. This is supposed to happen. Wait for the cake to fully cool before serving. If desired, top the cake with fresh whipped cream and chocolate curls.
Notes
For the whipped cream frosting, add 1 cup of cold heavy cream and 3 tbsp of sugar to a large mixing bowl and beat on highest speed until soft peaks form. Add dollops of soft whipped cream to the center of the cooled cake.
Recipe adapted from Richard Sax's chocolate cloud cake.
Estimated nutrition does not include optional whipped cream frosting.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.