SUNSHINE CAKE WITH CITRUS BUTTERCREAM

Steps:

• Make 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.