Step 1: Prepare the Cake Base
Preheat your oven to 325°F (160°C). Grease a 9-inch (24 cm) round springform cake pan and line the bottom with parchment paper.
In a heatproof bowl, melt the chopped chocolate and butter over a pot of simmering water, ensuring the bottom of the bowl doesn’t touch the water. Stir occasionally until melted, then set aside to cool slightly.
Step 2: Make the Cake Batter
In the bowl of an electric stand mixer fitted with the whisk attachment, beat the eggs, 4 egg yolks, granulated sugar, and vanilla on medium-high speed for 3-4 minutes, until light and fluffy.
In a separate bowl, combine the flour, baking powder, and ground hazelnuts. Mix well.
Gently fold the cooled chocolate mixture into the egg mixture, then fold in the flour mixture until just combined.
Pour the cake batter into the prepared cake pan and bake for 30-35 minutes. Remove the cake from the oven and set aside.
Step 3: Make the Chocolate Meringue
Increase the oven temperature to 350°F (175°C).
In the bowl of an electric mixer fitted with the whisk attachment, whisk the egg whites on medium-low until soft peaks form.
Gradually add the sugar in a slow, steady stream while whisking, then add the vinegar. Increase the speed to high and continue whisking for 2-3 minutes until the meringue is thick and glossy.
Combine the sifted cornstarch and cocoa powder, then gently fold them into the meringue mixture.
Step 4: Bake the Meringue
Spoon the chocolate meringue on top of the partially cooked cake.
Bake for an additional 20-25 minutes, or until the meringue is crisp and cooked through.
Let the cake cool in the pan for 15-20 minutes. Run a knife around the edge of the cake, then gently remove the springform pan.
Step 5: Chill and Serve
Allow the cake to cool completely before refrigerating for at least 2 hours.
Dust the cake with cocoa powder before serving.