- 1 1/2 cups walnuts (5 1/2 ounces), toasted and cooled
- 1/2 cup matzo cake meal
- 8 large eggs, separated, at room temperature for 30 minutes
- 1 1/3 cups sugar
- 1/2 teaspoon salt
- 2 teaspoons finely grated fresh orange zest
- 1/4 cup fresh orange juice
- 1 teaspoon vanilla
- 4 ounces bittersweet chocolate (not unsweetened), coarsely grated using 1/4-inch teardrop-shaped holes of a box grater
- Special equipment: a 10- by 2-inch springform pan
- Accompaniment: lightly sweetened whipped cream (optional)
- Put oven rack in middle position and preheat oven to 350°F.
- Insert bottom of springform pan upside down (so that turned-up edge faces down for easier removal of cake).
- 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). Add remaining 5 tablespoons matzo cake meal and pulse until mixture resembles a grainy flour.
- Beat egg whites with a pinch of salt in a bowl with an electric mixer at medium-high speed until they form soft peaks. Add 2/3 cup sugar a little at a time, beating until whites just hold stiff peaks.
- 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. Fold in nut flour, then zest, juice, and vanilla. Fold grated chocolate into batter gently but thoroughly. Fold in one third of whites to lighten batter, then fold in remaining whites gently but thoroughly.
- 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. Cool in pan on a rack 15 minutes, then run a thin sharp knife between cake and side of pan. Remove side of pan and cool cake completely. Invert cake onto rack and run knife between cake and bottom of pan to remove bottom, then flip onto a plate.