Preheat oven to 350F
Separate the eggs yolks from the egg whites and set aside to come to room temperature
In a large heat proof bowl add chocolate and butter. Microwave for 30 seconds then stir. Microwave for another 30 seconds and stir again. Repeat until chocolate and butter are melted and silky smooth
Add in egg yolks and whisk. Add in vanilla, cocoa powder and salt. Mix with a spatula until well combined and there are no lumps
Start beating the egg whites using an electric or stand mixer. Slowly add in the sugar in a few batches. Mix to medium stiff peaks
Add the whipped egg whites to the chocolate mixture in a few batches. Carefully fold in the egg whites making sure not to over mix
Pour the batter into a 9-inch baking pan lined with parchment paper. Smooth out with a spatula
Bake for 30 minutes. The cake will rise and for a crust at the top
Cake will come out of the oven tall and puffy. Allow to cool for 45 minutes. The center of the cake will deflate and form a lot of cracks. Serve with a dusting of powdered sugar, ice cream or fresh fruit
