6 ounces dark chocolate, chopped
8 tablespoons (112 g) unsalted butter, chopped (plus more for greasing)
2 egg whites (50 g) + 3 eggs (150 g, weighed out of shell)
3/4 cup (150 g) granulated sugar
1/2 cup (40 g) unsweetened cocoa powder, sifted (Dutch-processed is best)
1 teaspoon pure vanilla extract
1/2 teaspoon kosher salt
Preheat your oven to 350°F. Butter an 8-inch or 9-inch round springform pan, line just the bottom with a round of unbleached parchment paper, then butter the paper. Set the pan aside.
Melt the chocolate & butter. In a double boiler or a medium-sized microwave safe bowl, melt the chocolate and 8 tablespoons butter until both are melted and smooth. If you are using the microwave method, microwave in 30-second increments at 70% power, stirring in between intervals, taking care not to burn the chocolate. Set the mixture aside to cool briefly.
Beat the egg whites. In a stand mixer fitted with the whisk attachment (or in a large bowl using a hand mixer), beat the 2 egg whites on medium-high speed until soft peaks form. (For a slightly fluffier cake, separate one of the 3 whole eggs and add that white here.) Add about half of the sugar and about half of the cocoa powder slowly, beating on medium speed until glossy peaks begin to form. Adding cocoa powder to the egg whites makes it easier to incorporate them into the rest of the mixture later without deflating the whites.
Assemble the batter. In a large bowl, whisk the 3 whole eggs (or 2 whole eggs and remaining egg yolk), remaining sugar, remaining cocoa powder, vanilla and salt until well-combined. Add the melted chocolate and butter mixture slowly, whisking constantly, until fully combined. Continue to whisk until the mixture is smooth and glossy. Fold the egg white mixture into the batter carefully but completely. The mixture will be thickly pourable. Pour the batter into the prepared pan, and smooth the top carefully. Tap the pan gently on the counter to break any very large air bubbles.
Bake. Place the pan in the center of the preheated oven. Bake for about 25 minutes (closer to 30 minutes for an 8-inch cake), or until the cake is just set in the center. If you are using a dark-colored springform pan, begin checking at 20 minutes to ensure that cake doesn’t burn. Remove the pan from the oven and place the cake on a wire rack, still in the pan, to cool completely. Chill the cake in the refrigerator for a few minutes before unmolding it from the springform pan. Slice with a sharp, wet knife and serve.
This recipe was originally posted on the blog in 2013. The recipe method was slightly updated and the photos were redone in 2016, the video was added in 2017.