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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.
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2
Pulse 1 1/2 cups sugar and zest in food processor until no zest strands remain and sugar is damp and yellow. Alternatively, rub the zest into the sugar with fingertips.
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3
Whisk flour, baking powder, baking soda, and salt together in medium bowl; reserve. Combine milk, lemon juice, and vanilla in glass measuring cup; reserve.
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4
In large bowl, whip egg whites with whisk attachment 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. If you used the stand mixer bowl, transfer whites to separate bowl.
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5
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. Fold in egg whites and divide batter equally between prepared pans.
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6
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, then invert so the tops are facing up. Cool completely, about 1 hour.
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7
Pulse the zest and sugar in food processor until no zest strands remain and sugar is damp and yellow. Alternatively, rub the zest into the sugar with fingertips.
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8
Whisk the lemon sugar, cornstarch, eggs, egg yolks, and salt together in a medium heavy-bottomed saucepan. Add juice and cook over medium-low heat, whisking constantly, until thickened and the consistency of pudding, 8 to 10 minutes. Once mixture reaches the boil, continue cooking and whisking for 2 minutes longer.
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9
Strain curd into medium bowl. Add butter and stir until completely incorporated. Place a piece of plastic wrap directly on the surface of the custard and refrigerate until completely chilled. Alternatively, place bowl in ice bath and stir constantly until custard is completely cooled, about 10 minutes.
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10
With a serrated knife, cut each cake in half to create 4 equal layers. Place one layer on a serving dish or cake stand and spread with one third (about 1/2 cup) of cooled curd. Repeat with remaining cake layers and curd.
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11
Sift confectioners' sugar over cake. Serve.
