Preheat oven to 350°F (175°C). Grease a 9-inch springform pan (at least 3 inches deep). Line with parchment, and grease the parchment.
Using an electric mixer on medium speed, beat the butter and brown sugar until light and fluffy. Add the 6 egg yolks, one at a time, beating well after each addition.
Add the melted chocolate, vanilla, and liqueur. Mix until combined.
In a separate mixing bowl, combine the 6 egg whites and the salt. Using an electric mixer with a whisk attachment, beat on high speed until soft peaks form (about 2 minutes).
Stir about a third of the egg whites into the chocolate mixture. Then gently fold in the remaining egg whites, mixing just until combined.
Transfer the batter to the prepared pan, and spread evenly. Place the pan on a lined, rimmed baking sheet to catch any spills. Bake 25 to 30 minutes, or until the center is not quite set.
To make the meringue:
Combine the hazelnuts, chopped chocolate, and cornstarch. Set aside.
Using an electric mixer with a whisk attachment, beat the 4 egg whites and cream of tartar on high speed until the mixture is frothy. While continuing to beat the mixture, slowly add the sugar. Continue beating until stiff peaks form (about 8 minutes).
Gently fold in the hazelnut mixture.
Spread the meringue mixture evenly on top of the partially baked cake.
Bake 25 to 30 minutes, or until the meringue is lightly browned and crisp.
Cool the cake on a wire rack for 10 minutes. Then run a knife around the edge of the pan and release the sides of the pan. Cool the cake for at least 30 minutes before serving.
Notes
The original recipe uses rum. A hazelnut liqueur would also be a great choice.
I like to take a shortcut and use blanched hazelnuts. If you use hazelnuts with skins, rub them with a kitchen towel after they've been toasted to remove the skins before chopping them.
Superfine sugar will work best for the meringue because it will incorporate into the mixture more easily and create a better texture. If you don't have access to superfine sugar, then you can approximate it by processing a cup plus about 2 teaspoons of granulated sugar in a food processor for about 30 seconds.