Step 1

Preheat the oven to 350°F (175°C). Line the bottom of an 8-inch springform pan with a round of wax paper.

Step 2

Melt the chocolate in a bowl set over but not touching gently simmering water. Remove the bowl from the heat and whisk in the butter until it's melted and completely incorporated.

Step 3

In another bowl, whisk the 2 whole eggs and the 4 egg yolks with 1/2 cup of the sugar until foamy and well combined. Slowly whisk in the warm chocolate mixture. Whisk in the Cognac or Grand Marnier and the orange zest, if using. In another bowl, with an electric mixer, beat the 4 egg whites until really quite foamy. Gradually add the remaining 1/2 cup sugar and beat until the whites form soft peaks that hold their shape but aren't stiff when you remove the beaters. Gently, gently fold about 1/4 of the beaten egg whites into the chocolate mixture to lighten it, then carefully fold in the remaining whites. Pour the batter into the pan and smooth the top.

Step 4

Bake the cake until the top is puffed and cracked and the center is no longer wobbly, 35 to 40 minutes. Be careful not to overbake the cake.

Step 5

Cool the cake in the pan on a wire rack. The center of the cake will sink as it cools, forming a sort of crater.

Step 6

When ready to serve, whip the cream with the confectioners' sugar and vanilla until not quite stiff. Using a spatula, carefully fill the sunken center of the cake with the whipped cream, pushing the billowy cream gently all the way to the edges of the cake in decorative swoops and swirls. Sprinkle the top lightly with cocoa powder. Run the tip of a knife around the edge of the cake, carefully remove the side of the pan, and serve.