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.