• 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 large bowl with a handheld mixer, or a stand mixer fitted with the whisk attachment, 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 one more white here.)
• Add about half of the sugar and about half of the cocoa powder to this mixture, and beat on medium speed until well-combined and glossy. Go slowly at first, because cocoa powder resists combining.
• 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 separate 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, whisking constantly, until fully combined. Continue to whisk until the mixture is smooth and glossy.
• Fold the egg white and cocoa powder/sugar 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 using a small offset spatula or knife. Tap the pan gently on the counter to break any very large air bubbles.

Bake the cake.

• 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 the 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. It should take about an hour to cool in the pan fully.
• Place the cooled cake in the refrigerator, still in the pan, and chill for 1 hour (or longer) before unmolding it from the springform pan.
• Slice with a sharp, wet knife and serve with optional cocoa powder, berries and cream.