Only serious chocaholics need apply. Thanks Baker By Nature!
12 tablespoons unsalted butter
12 ounces bittersweet chocolate, chopped into small pieces
6 large eggs, at room temperature, divided
1 tablespoon pure vanilla extract
1/4 cup natural, unsweetened cocoa powder
3/4 teaspoon salt
1 cup sugar, divided
Preheat oven to 350 degrees (F). Grease and flour a 9″ springform pan; set aside. Combine the butter and chocolate together in a small pan over medium-low heat; cook, stirring almost constantly, until both are completely melted. Whisk smooth then remove pan from heat; set aside.
Separate 4 of the 6 eggs, placing the egg whites and yolks in separate medium-sized bowls. In the bowl with with yolks add the vanilla extract, cocoa powder, salt, remaining eggs, and half of the sugar; whisk well to combine. Slowly add the cooled chocolate mixture into the yolk mixture; whisk well and set aside.
Using an electric mixer set to medium-high speed, beat egg whites until very frothy. Gradually beat in the remaining 1/2 cup sugar and beat until firm peaks form.
Carefully fold the egg whites into the chocolate mixture, stirring until incorporated. Pour the batter into the prepared pan and smooth top with a spatula.
Place pan in oven and bake until the top is slightly cracked and the middle no longer moves, about 35 to 40 minutes. Transfer cake to cooling wrack and let cool completely before cutting and serving.