INGREDIENTS
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10 tablespoons unsalted butter, plus more for pan
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1 cup hazelnuts
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All-purpose flour for pan
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3/4 cup firmly packed light-brown sugar
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6 large whole eggs, separated
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4 large egg whites
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12 ounces bittersweet chocolate, melted and cooled, plus 4 ounces roughly chopped
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1 tablespoon pure vanilla extract
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1 tablespoon rum, optional
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Pinch of salt
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1 tablespoon cornstarch
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1/4 teaspoon cream of tartar
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1 cup superfine sugar