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1
Make cake layers: Preheat oven to 350u00b0F Butter 2 (8- by 2-inch) round cake pans and dust with flour, knocking out excess.
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2
Put sugar and zests in a food processor and pulse until blended well, about 30 seconds. Sift together flour, baking powder, baking soda, and salt.
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3
Beat butter in large bowl of standing electric mixer until light and fluffy, about 2 minutes.
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4
Gradually beat in sugar mixture and beat at high speed 5 minutes. Beat in egg yolks, 1 at a time, and beat until light and fluffy, about 4 minutes.
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5
Add one fourth of flour mixture and beat on low speed until just combined. Beat in milk until just blended.
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6
Beat in remaining flour mixture alternately with juices in 3 batches, beginning and ending with flour mixture.
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7
Divide batter evenly between cake pans and smooth tops. Bake in middle of oven until golden and a tester comes out clean, about 30 minutes.
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8
Cool layers on racks 5 minutes. Run a thin knife around edges of pans, then invert cakes onto racks to cool completely.
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9
Make buttercream: Heat whites and sugar in a metal bowl set over a saucepan of simmering water, whisking constantly, until sugar is dissolved and a thermometer registers 160u00b0F
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10
Remove bowl from heat and beat mixture in standing electric mixer on medium-high speed until thick, glossy peaks form. If mixture is still warm, continue beating until cool.
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11
With mixer running, add butter, 1 tablespoon at a time, beating well after each addition.
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12
Add liqueur and lemon juice, beating on high speed until smooth and fluffy, about 10 minutes. If buttercream begins to separate, beat on high speed until smooth.
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13
Assemble cake: Brush any loose crumbs from layers and put 1 upside down on a serving plate.
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14
Spread with about 1 cup buttercream. Place other layer on top, right side up. Frost top and sides of cake with remaining buttercream.
