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1
Preheat oven to 350F.
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2
Butter a 15- by 10- by 1-inch baking pan and line bottom with wax or parchment paper.
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3
Butter paper.
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4
Pulse walnuts in a food processor until finely chopped (do not allow to become a paste) and stir together with flour in a bowl.
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5
Beat together butter and 1/3 cup sugar in a large bowl with an electric mixer until pale and fluffy.
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6
Add yolks 1 at a time, beating well after each addition, then beat in vanilla.
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7
Stir in nut mixture by hand.
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8
Beat egg whites with salt with cleaned beaters in another bowl at medium speed until they hold soft peaks.
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9
Add remaining 1/3 cup sugar and beat at high speed until whites just hold stiff peaks.
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10
Stir one fourth of whites into batter to lighten, then fold in remaining whites gently but thoroughly.
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11
Spread batter evenly in baking pan, smoothing top.
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12
Bake in middle of oven until center is set and top is pale golden, 25 to 30 minutes.
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13
Cool cake in pan on a rack.
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14
Cover cake with an inverted baking sheet, then invert cake onto it and remove paper.
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15
Reinvert cake onto a cutting board.
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16
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).
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17
Line baking sheet with wax or parchment paper and transfer cake to baking sheet, arranging layers side by side, touching.
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18
Sprinkle gelatin over water in a small bowl and let stand 1 minute to soften.
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19
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.
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20
Remove bowl from heat and set in a larger bowl of ice water.
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21
Cool, stirring occasionally, until cold, 5 to 10 minutes.
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22
Spread jam evenly over cake layers and chill until set, about 1 hour.
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23
Reserve the nicest-looking cake layer for top.
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24
Put 1 cake layer, jam side up, on a cake platter and spread with 1/3 cup buttercream.
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25
Top with second cake layer, jam side up, and spread with 1/3 cup buttercream.
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26
Top with third layer, jam side up, and spread with 1/3 cup buttercream.
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27
Top with reserved cake layer, jam side up.
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28
Frost sides of cake with about 1 cup buttercream.
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29
Transfer remaining buttercream to pastry bag and pipe a decorative border around top edge of cake.