- 1 1/2 cups walnuts (6 oz)
- 1/3 cup all-purpose flour
- 1 stick (1/2 cup) unsalted butter, softened
- 2/3 cup sugar
- 6 large eggs, separated
- 1 teaspoon vanilla
- 3/4 teaspoon salt
- 1 1/2 teaspoons unflavored gelatin (less than 1 envelope)
- 1 tablespoon water
- 3/4 cup strained blackberry preserves (from 1 cup with seeds)
- Mocha buttercream
- Special equipment: a pastry bag fitted with an 3/4-inch plain tip
- Preheat oven to 350°F. Butter a 15- by 10- by 1-inch baking pan and line bottom with wax or parchment paper. Butter paper.
- Pulse walnuts in a food processor until finely chopped (do not allow to become a paste) and stir together with flour in a bowl.
- Beat together butter and 1/3 cup sugar in a large bowl with an electric mixer until pale and fluffy. Add yolks 1 at a time, beating well after each addition, then beat in vanilla. Stir in nut mixture by hand.
- Beat egg whites with salt with cleaned beaters in another bowl at medium speed until they hold soft peaks. Add remaining 1/3 cup sugar and beat at high speed until whites just hold stiff peaks. Stir one fourth of whites into batter to lighten, then fold in remaining whites gently but thoroughly.
- Spread batter evenly in baking pan, smoothing top. Bake in middle of oven until center is set and top is pale golden, 25 to 30 minutes. Cool cake in pan on a rack. Cover cake with an inverted baking sheet, then invert cake onto it and remove paper.
- Reinvert cake onto a cutting board. Trim about 1/4 inch from each side of cake with a sharp knife, then cut cake crosswise into 4 equal rectangles (each about 9 by 3 1/2 inches). Line baking sheet with wax or parchment paper and transfer cake to baking sheet, arranging layers side by side, touching.
- Sprinkle gelatin over water in a small bowl and let stand 1 minute to soften. Heat strained preserves in a double boiler or a metal bowl set over a saucepan of simmering water until warm, then add gelatin mixture and stir until dissolved, about 2 minutes. Remove bowl from heat and set in a larger bowl of ice water. Cool, stirring occasionally, until cold, 5 to 10 minutes.
- Spread jam evenly over cake layers and chill until set, about 1 hour.
- Reserve the nicest-looking cake layer for top. Put 1 cake layer, jam side up, on a cake platter and spread with 1/3 cup buttercream. Top with second cake layer, jam side up, and spread with 1/3 cup buttercream. Top with third layer, jam side up, and spread with 1/3 cup buttercream. Top with reserved cake layer, jam side up. Frost sides of cake with about 1 cup buttercream. Transfer remaining buttercream to pastry bag and pipe a decorative border around top edge of cake.