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1.
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Preheat oven to 350 degrees.
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Butter a 9 x 3 inch springform pan.
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Line bottom with parchment.
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Butter parchment, and sprinkle with flour; tap out excess.
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Set aside.
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2.
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Spread hazelnuts in a single layer on a rimmed baking sheet.
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Toast until fragrant and skins start to crack, 10-15 minutes.
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Remove from oven; rub vigorously with a clean kitchen towel to take off skins.
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Let cool; roughly chop.
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Set aside.
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3.
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Make cake batter; In the bowl of an electric mixer fitted with the paddle attachment, cream butter and brown sugar until pale and smooth.
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Add egg yolks, one at a time, beating well after each addition, until mixture is light and fluffy.
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Add melted chocolate, vanilla, and rum.
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Beat until combined.
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Set aside.
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4.
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In a clean mixer bowl, combine 6 egg whites and salt; using the whisk attachment, beat on high speed until soft peaks form, about 2 minutes.
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Stir one third of the egg whites into chocolate mixture.
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Fold in remaining beaten egg whites just until combined.
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Pour batter into prepared pan, and bake 25 minutes.
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5.
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Meanwhile, make meringue; Combine hazelnuts, chopped chocolate, and cornstarch in a small bowl, and set aside.
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Place remaining 4 egg whites and cream of tartar in a clean mixer bowl; using a clean whisk attachment beat on high speed until frothy.
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With mixer running, slowly add superfine sugar; continue beating until stiff peaks form, about 8 minutes.
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Fold in hazelnut mixture.
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6.
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Remove cake from oven.
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Using an offset spatula, spread meringue mixture on top of cake, and return to oven.
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Bake until meringue is lightly browned and crisp, 25-30 minutes.
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Transfer pan to a wire rack; let stand 10 minutes.
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Run a knife around the edge of the cake to loosen, and release sides of pan.
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Let cool, about 30 minutes, before slicing and serving.
