12ouncesbittersweet chocolate, chopped into small pieces
6large eggs, at room temperature, divided
1tablespoonpure vanilla extract
1/4cupnatural, unsweetened cocoa powder
3/4teaspoonsalt (fine sea salt or kosher salt will work)
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.
Dust the cake with cocoa powder, confectioners' sugar, or top with a dollop of whipped cream and fresh berries!
This cake MUST cool completely before being sliced. To speed this up you can cool the cake on the counter for 30 minutes, then pop it in the fridge for another 20 minutes or so. If you slice into the cake before it's cooled, it will be too gooey in the middle. The cake sets as it cools.