CITRUS MOUSSE CAKE WITH BUTTERCREAM FROSTING

Steps:

• Preheat oven to 325 degrees. Butter two 8-by-2-inch round cake pans. Line with parchment; butter parchment. Dust with flour, tapping out excess.
• Whisk together flour, baking powder, baking soda, and salt until thoroughly combined, 30 seconds. Beat butter with sugar and zest until light and fluffy. Beat in eggs, one at a time, thoroughly combining after each addition and scraping down bowl as needed. Beat in lemon juice. With mixer on low, add flour mixture in 3 batches, alternating with buttermilk and beginning and ending with flour.
• Divide batter evenly between prepared pans, smoothing tops with an offset spatula. Bake, rotating pans halfway through, until tops spring back when lightly touched and are light golden, about 1 hour, 10 minutes. Transfer pans to wire racks; let cool 10 minutes. Turn out of pans, remove parchment, and let cool completely, about 2 hours. Cakes can be made to this point and refrigerated, well wrapped in plastic, up to 1 day.
• Trim tops of cakes flat. Split each cake horizontally into 3 equal layers. Line an 8-inch cake pan with 2 parchment strips, leaving a 3-inch overhang on all sides. Place 1 cake layer in pan, bottom-side down. Spread 1 cup mousse evenly over it; top with a second cake layer. Repeat, spreading 1 cup mousse between each layer. Finish with a cake layer, bottom-side up. Refrigerate, covered in plastic, at least 1 hour and up to overnight.
• Turn cake out of pan onto a cake plate. Spread 1 cup buttercream evenly over top and sides to create a crumb coat. Refrigerate 15 minutes. Spread remaining 3 cups buttercream over top and sides. Decorate as desired.