Sunshine Cake with Citrus Butter Cream

PreparationMake cake layers: Preheat oven to 350°F. Butter 2 (8- by 2-inch) round cake pans and dust with flour, knocking out excess. 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. Beat butter in large bowl of standing electric mixer until light and fluffy, about 2 minutes. 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. Add one fourth of flour mixture and beat on low speed until just combined. Beat in milk until just blended. Beat in remaining flour mixture alternately with juices in 3 batches, beginning and ending with flour mixture. 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. Cool layers on racks 5 minutes. Run a thin knife around edges of pans, then invert cakes onto racks to cool completely. 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 160°F. 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. With mixer running, add butter, 1 tablespoon at a time, beating well after each addition. 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. Assemble cake: Brush any loose crumbs from layers and put 1 upside down on a serving plate. Spread with about 1 cup buttercream. Place other layer on top, right side up. Frost top and sides of cake with remaining buttercream. Cooks' note:Cake may be made 1 day ahead and chilled, loosely covered. Bring to room temperature before serving.