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1
Heat oven to 350u00b0F. with rack in middle. Butter cake pans, then line bottoms with rounds of parchment. Dust pans with additional flour, knocking out excess.
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2
Sift flour, baking powder, and salt together into a bowl.
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3
Beat together butter and sugar in a large bowl with an electric mixer (or a stand mixer fitted with the paddle attachment) at medium-high speed until pale and fluffy, about 3 minutes. Beat in eggs one at a time, beating well after each addition. Beat in zest and vanilla and continue beating until light and fluffy, 3 to 5 minutes more. At low speed, add flour mixture in 3 batches alternately with milk, beginning and ending with the flour mixture and mixing until batter is just smooth.
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4
Divide batter between cake pans, spreading it evenly. Bake until a wooden toothpick inserted in the center of each cake comes out clean and cakes begin to pull away from sides of pans, 20 to 25 minutes.
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5
Cool for 15 minutes in pans on racks. Run a thin knife around sides of pans, invert cake layers onto racks, and cool completely.
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6
Whisk together sugar, cornstarch, and salt in a heavy 2-quart saucepan, then whisk in 3/4 cup water and juice until smooth. Whisk together eggs in a medium heat-proof bowl until well combined.
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7
Bring juice mixture to a boil over medium heat, stirring with whisk, then reduce heat and continue to cook at a slow boil, stirring with whisk, 1 minute (mixture will be thick). Remove pan from heat.
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8
Add half of hot juice mixture to eggs in a slow stream, stirring with whisk. Stir egg mixture into juice in pan and cook over medium-low heat, stirring, just until it reaches a simmer. Remove pan from heat and add butter and zest, stirring until butter is melted.
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9
Cover filling with a buttered round of wax paper or parchment paper and chill until cold, about 1 hour.
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10
Combine frosting ingredients in a large bowl with electric mixer (or stand mixer fitted with the whip attachment). Beat at high speed until frosting is thick and fluffy, 3 to 5 minutes.
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11
Halve each cake horizontally with a long serrated knife. Put 1 layer on a cake stand or a large plate and spread with one-third of filling (about 2/3 cup).
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12
Stack remaining cake layers using half of remaining filling between each layer.
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13
Spread top and side of cake generously with frosting.
