Mixed berry and White Chocolate Buttercream Cake

24 Servings. Position 1 rack in center and 1 rack in lowest third of oven and preheat to 350 degrees F. Butter and flour two 12-inch-diameter cake pans with 2-inch-high sides. Whisk flour, 1 1/2 cups sugar baking powder and salt in very large (6- to 8-quart) bowl to blend well. Add 1 1/2 cups lukewarm water; using electric mixer, beat until smooth (batter will be thick). Beat in oil, then egg yolks and vanilla. Using clean dry beaters, beat egg whites and powdered sugar in large bowl until whites are very thick and billowy and form medium-firm peaks when beaters are lifted, about 12 minutes (do not underbeat). Fold whites into yolk mixture in 3 additions. Divide cake batter equally between prepared pans. Place one pan on each rack in oven, staggering pans so that one is not directly above the other. Bake cakes 25 minutes. Reverse positions of pans. Bake cakes until tester inserted into center comes out clean, about 10 minutes. Cool cakes in pans 10 minutes. Line 2 cooling racks with kitchen towels. Cut around cakes to loosen; turn out onto towels. Cool completely. Buttercream Makes about 8 cups 8 ounces good-quality white chocolate (Lindt, Baker's or Callebaut), chopped 1/2 cup whipping cream 2 cups (4 sticks) unsalted butter, room temperature 1 cup egg whites (about 8) 2 1/4 cups sugar 2 1/2 teaspoons grated orange peel 2 teaspoons vanilla extract Stir white chocolate and cream in heavy small saucepan over low heat until melted and smooth. Cool to room temperature. Using electric mixer, beat butter in medium bowl until fluffy; set aside. Combine whites and sugar in large metal bowl. Set bowl over large saucepan of simmering water (do not let bottom of bowl touch water). Whisk until sugar dissolves and thermometer inserted into whites registers 160 degrees F, about 3 minutes. Remove bowl from over water. Using clean dry beaters, beat meringue until cool, stiff and shiny, about 10 minutes. Beat butter, about 1/4 cup at a time, into meringue, blending well after each addition. If frosting looks curdled, place bowl over very low heat for 3 to 5 seconds to soften slightly. Remove from heat and beat until smooth. Repeat warming technique as necessary. Gradually beat in cooled white chocolate mixture, then orange peel and vanilla extract. Chill buttercream until beginning to firm, about 30 minutes. Assembly 1 cup seedless raspberry jam 6 cups (about) assorted fresh berries (raspberries; blueberries; blackberries, halved if large; and sliced strawberries) Cut each cake in half horizontally. Place 1 layer on 11-inch-diameter tart pan bottom or cardboard round. Spread with 1/2 cup jam, then 1 1/4 cups buttercream. Arrange half of berries atop buttercream spacing about 1/2 inch apart. Press second cake layer atop berries. Repeat layering of jam, buttercream and remaining berries. Top with third cake layer (reserve remaining cake layer for another use). Spread 2 cups buttercream in thin layer over top and sides of assembled cake to anchor crumbs and smooth surface. Refrigerate cake on its base until crumb coat is firm, about 30 minutes. Spread 2 1/4 cups buttercream over top and sides of cake. Spoon remaining buttercream into pastry bag fitted with medium star tip. Pipe decorative border around top edge of cake. 4 cups (about) assorted fresh berries (blueberries, raspberries, blackberries and strawberries) 1/2 cup currant jelly Place cake with its base on platter. Fill top center of cake with berries. Melt jelly in heavy small saucepan over low heat, stirring constantly. Using pastry brush, brush warm jelly over tops of most berries. Let cake stand at room temperature at least 1 hour and up to 3 hours. Using long knife, cut cake into quarters. Cut each quarter into 6 wedges.