01 -
Preheat oven to 350°F (175°C). Grease and flour a 9-inch springform pan; set aside.
02 -
Combine the butter and chocolate in a small pan over medium-low heat; cook, stirring almost constantly, until completely melted. Whisk until smooth then remove from heat; set aside to cool slightly.
03 -
Separate 4 of the 6 eggs, placing the egg whites and yolks in separate medium-sized bowls. In the bowl with yolks, add the vanilla extract, cocoa powder, salt, remaining whole eggs, and half of the sugar; whisk well to combine.
04 -
Slowly add the cooled chocolate mixture into the yolk mixture; whisk well and set aside.
05 -
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 continue beating until firm peaks form.
06 -
Carefully fold the egg whites into the chocolate mixture, stirring until fully incorporated. Pour the batter into the prepared pan and smooth top with a spatula.
07 -
Place pan in oven and bake until the top is slightly cracked and the center no longer moves, about 35 to 40 minutes.
08 -
Transfer cake to cooling rack and let cool completely before cutting and serving.
09 -
Dust the cake with cocoa powder, confectioners' sugar, or top with a dollop of whipped cream and fresh berries.