Preheat oven to 350 degrees. Butter a 9-by-3-inch springform pan. Line bottom with parchment paper. Butter parchment and set aside.

In the bowl of an electric mixer fitted with the paddle attachment, whip the butter and brown sugar until pale and smooth at medium speed, about 3 minutes. Add egg yolks, one at a time, beating well after each addition and scraping the bottom and sides of your bowl if necessary. Add the melted chocolate, espresso (or vanilla), rum, if using, and salt. Beat until combined. Transfer to a clean bowl and wash your mixing bowl thoroughly.

In a clean mixer bowl fitted the whisk attachment, beat the 6 egg whites on high speed until soft peaks form, about 2 minutes. Fold one-third of the egg whites into chocolate mixture. Fold in remaining egg whites and pour batter into prepared pan, and bake 25 minutes.

While cake base is baking, prepare the meringue. Combine the chopped chocolate, nuts and cornstarch in a small bowl, and set aside. Place remaining 4 egg whites in a clean mixer bowl fitted with the whisk attachment, beat on high speed until frothy. With the mixer running, slowly add the sugar and continue beating until stiff peaks form, about 8 minutes. Fold in hazelnut mixture.

After 25 minutes, remove cake from oven. Using a large offset spatula, quickly spread meringue mixture on top of cake using as little strokes as possible not to deflate the meringue, and return to oven. Bake until meringue is lightly browned and crisp, 25 to 35 minutes. Transfer pan to a wire rack; let stand 10 minutes. Run a knife around the edge of the cake to loosen, and release sides of pan. Let cool, about 30 minutes, before slicing and serving.