Walnut-Blackberry Torte with Mocha Buttercream

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.