Preheat oven to 350F and line the bottom of an 8" springform pan with parchment paper. Do not grease.
Melt the chocolate in a heatproof bowl over a simmering pot of water until smooth.
8 oz (225g) best-quality bittersweet or semi-sweet chocolate
Stir in the butter until completely melted and combined. Set aside.
1/2 cup (114g) unsalted butter
In a large mixing bowl, whisk together the 4 egg yolks along with the 2 whole eggs and 1/2 cup sugar.
6 large eggs, 1 cup (200g) granulated sugar
Slowly whisk the melted chocolate into the egg yolk mixture until nice and smooth. Set aside.
In another large mixing bowl, beat the egg whites until frothy. Gradually add in the remaining 1/2 cup sugar and continue beating until soft peaks form.
Very gently fold 1/4 of the meringue at a time into the chocolate mixture until just combined. Transfer the batter into the prepared pan.
Bake for 35-45 minutes or until puffed up and cracked and the center is no longer wobbly. Be careful not to overbake.
Let cool completely on a wire rack. As the cake cools it will collapse and cave in. Do not remove from pan until ready to serve.
Beat the whipping cream powdered sugar and vanilla until soft peaks form. Fill the sunken center of the cake with the whipped cream. Dust with cocoa powder and top with chocolate shavings.