Preheat your oven to 350 degrees Fahrenheit. Butter and flour a 9-inch springform pan, and set aside.
Place 6 oz dark chocolate in a microwave safe bowl and microwave in 30 second increments until the chocolate is melted, stirring between each set of 30 seconds. Set aside.
In a large mixing bowl, use an electric hand mixer to cream ⅔ cup butter and ½ cup of sugar together for about 3 minutes until it is pale, fluffy, and creamy.
Add the 7 egg yolks one at a time, blending after each addition. Add 2 tsp vanilla.
Use a rubber spatula to fold in the melted chocolate, 1 cup flour, and ¼ tsp salt. Set aside.
In a separate bowl, beat the 7 egg whites for about a minute. Add the remaining ½ cup of sugar and then mix on high speed until stiff peaks form (about 5-8 more minutes).
Carefully fold the 7 egg whites into the batter in 3 parts. Mix until there are no streaks of egg whites. Do not overmix as you don’t want to deflate the egg whites.
Pour the batter into the prepared pan and bake for 50-60 minutes, until a toothpick inserted in the center of the cake comes out clean. Allow the cake to cool in the pan for at least an hour.