In a microwave safe bowl add chocolate and butter. Melt in 30 second increments whisking in between until creamy and smooth.
Whisk in sugar, vanilla, salt, and the six egg yolks. Set aside.
In a separate bowl whip egg whites with electric mixer until stiff peaks form. Gently fold egg whites with chocolate mixture until combined and smooth.
Pour batter into a 9 inch spring form pan. Bake for 1 hour 15 minutes. Cool for at least one hour before removing from pan.