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1
Lightly coat two 9-inch round cake pans with baking spray. Line bottoms with parchment paper rounds and coat once again with baking spray. Adjust oven rack to middle position and preheat oven to 350u00b0F. In food processor, pulse 1 1/2 cups sugar and lemon zest until no zest strands remain. Whisk flour, baking powder, and baking soda together in medium bowl; reserve. Combine milk, lemon juice, and vanilla in glass measuring cup; reserve.
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2
In bowl of stand mixer fitted with the whisk attachment, whip egg whites and salt on medium speed until soft peaks form, about 2 minutes. Gradually add remaining 1/4 cup sugar and continue whipping until stiff peaks form, 1 to 2 minutes longer. Transfer whites to separate bowl.
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3
Beat butter and lemon sugar with paddle attachment on medium speed until light and fluffy, about 2 minutes. Add egg yolks, one at a time, beating well after each addition. Add flour mixture in three additions, alternating with milk mixture.
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4
Stir 1/3 of egg whites into batter until fully incorporated. With rubber spatula, fold in remaining egg whites. Divide batter equally between prepared pans.
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5
Bake until tester inserted in center of cakes comes out clean, 20 to 25 minutes. Transfer cakes to cooling racks and cool 10 minutes in pans. Turn cakes out directly onto racks, peel off and discard parchment paper, then invert so tops are facing up. Cool completely, about 1 hour.
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6
Pulse sugar and lemon zest in food processor until no zest strands remain. Bring lemon juice and lemon sugar to boil over medium-high heat in medium heavy-bottomed saucepan. Cook until reduced to 1/3 cup, 10 to 15 minutes.
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7
Combine flour, cornstarch, and salt in medium bowl. Whisk in milk until smooth. Strain mixture into medium saucepan and cook over medium heat, whisking constantly, until mixture comes to a boil and thickens, 5 to 7 minutes. Stir in lemon mixture, white chocolate, and vanilla. Transfer mixture to large bowl and cool to room temperature, stirring occasionally, 1 to 2 hours. Alternatively, place bowl in ice bath and stir constantly until cooled, about 10 minutes.
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8
Beat milk mixture on low speed with whisk attachment until combined. Add butter, 1 piece at a time, waiting for each piece to be incorporated prior to adding the next. Increase mixer speed to medium-high and beat until frosting is light and fluffy, about 5 minutes.
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9
With serrated knife, cut each cake in half. Arrange one cake layer on cake stand or serving plate. Stir blackberry jam to loosen, then, with offset spatula, spread 1 cup on cake. Press second cake layer on top and, with clean offset spatula, spread with about 3/4 cup icing. Repeat procedure with remaining 1 cup blackberry jam and cake.
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10
Ice top and sides of cake. Press cookies into sides of cake.
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11
Bring cream and shortening to simmer in small saucepan or in bowl in microwave. Place chocolate in medium bowl. Add cream and stir until smooth. Stir in lemon juice and salt.
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12
With a pastry brush, paint cookies with melted chocolate mixture. Once cookies are coated, repeat procedure until no chocolate remains.
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13
Refrigerate 20 to 30 minutes to set. Top cake with blackberries. Serve.
