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1
Put oven rack in middle position and preheat oven to 350F.
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2
Insert bottom of springform pan upside down (so that turned-up edge faces down for easier removal of cake).
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3
Pulse walnuts with 3 tablespoons matzo cake meal in a food processor until nuts are finely chopped (be careful not to pulse to a paste).
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4
Add remaining 5 tablespoons matzo cake meal and pulse until mixture resembles a grainy flour.
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5
Beat egg whites with a pinch of salt in a bowl with an electric mixer at medium-high speed until they form soft peaks.
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6
Add 2/3 cup sugar a little at a time, beating until whites just hold stiff peaks.
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7
Beat together yolks and remaining 2/3 cup sugar with 1/2 teaspoon salt in a large bowl at medium-high speed until thick and pale, about 3 minutes in a standing mixer or about 5 minutes with a handheld.
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8
Fold in nut flour, then zest, juice, and vanilla.
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9
Fold grated chocolate into batter gently but thoroughly.
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10
Fold in one third of whites to lighten batter, then fold in remaining whites gently but thoroughly.
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11
Pour batter into ungreased springform pan, smoothing top, and bake until a wooden pick or skewer inserted in center comes out clean, 50 minutes to 1 hour.
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12
Cool in pan on a rack 15 minutes, then run a thin sharp knife between cake and side of pan.
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13
Remove side of pan and cool cake completely.
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14
Invert cake onto rack and run knife between cake and bottom of pan to remove bottom, then flip onto a plate.
