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1
Preheat the oven to 350u00b0F and position a rack in the center. Lightly grease a 9-inch spring-form pan with nonstick cooking spray or butter, dust it with flour, and tap to knock out the excess.
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2
Place the walnut pieces on a baking sheet and toast them until they are golden brown and aromatic, 12 to 14 minutes.
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3
Allow the walnuts to cool completely, then place them in a food processor with the cocoa powder and pulse until the nuts are finely ground.
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4
Melt the chocolate and butter together in a large, heatproof bowl over a pan of simmering water, stirring often to prevent scorching.
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5
Remove the bowl from the heat and allow the mixture to cool to lukewarm.
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6
In an electric mixer fitted with the paddle attachment, beat the egg yolks with the light brown sugar on medium speed until very thick and pale in color, about 3 minutes.
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7
Beat in the rum and the 1/2 teaspoon salt.
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8
Using a spatula, fold the egg mixture into the cooled chocolate and butter mixture, then fold in the walnuts and cocoa.
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9
Place the egg whites in clean, dry, greaseless bowl adding the pinch of salt and, using the whisk attachment, beat on medium speed until the whites are thick but not peaking, about 2 minutes.
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10
Continue beating while you gradually add the granulated sugar, then switch to high speed, beating the whites until they are softly peaking, about 1 minute.
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11
Fold the whites into the cake batter, then immediately pour the batter into the prepared pan, smoothing the surface with a spatula.
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12
Bake the cake for 40 to 45 minutes, or until it is slightly puffed and just beginning to crack.
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13
Rotate the cake pan gently halfway through the baking time to ensure even browning.
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14
Allow the cake to cool in the pan for 20 minutes, then remove the sides of the pan and cool the cake completely; the center may sink a bit.
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15
Carefully remove the bottom of the spring-form pan and transfer the cake to a serving plate.
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16
Any leftover cake may be wrapped in plastic and served the following day.
