Preheat your oven to 350°F (175°C).
In a double boiler or microwave-safe bowl, combine the chopped chocolate and butter until smooth.
In a mixing bowl, whisk together the egg yolks, 6 tablespoons of sugar, and vanilla extract until pale and creamy.
Slowly add the chocolate mixture to the egg yolks while whisking continuously.
In a separate bowl, beat the egg whites until soft peaks form, then gradually add the remaining sugar until stiff peaks form.
Gently fold one-third of the whipped egg whites into the chocolate mixture, then fold in the remaining egg whites until no streaks remain.
Pour the batter into the prepared cake pan and bake for 25-30 minutes.
Let the cake cool in the pan for 10 minutes before inverting onto a cooling rack. Prepare the ganache by melting 6 ounces of chocolate with ½ cup of heavy cream.
Pour the ganache over the cooled cake and let it set before slicing.
