01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper, ensuring even coating for easy release.
02 - In a medium bowl, whisk together all-purpose flour, baking powder, baking soda, and salt until thoroughly incorporated. Set aside for later use.
03 - Using an electric mixer on medium-high speed, beat unsalted butter and granulated sugar until pale, light, and fluffy, approximately 3-4 minutes. Add eggs one at a time, mixing well after each addition. Blend in lemon zest and vanilla extract.
04 - With mixer on low speed, add flour mixture in three portions, alternating with buttermilk and lemon juice in two additions. Begin and end with flour mixture, stopping once just combined. Avoid overmixing to prevent tough texture.
05 - Divide batter evenly among prepared pans, smoothing tops. Bake for 25-30 minutes until golden and a wooden toothpick inserted in center comes out clean. Cool in pans for 10 minutes, then invert onto wire racks to cool completely before assembly.
06 - Whisk eggs and sugar in a heatproof bowl until smooth. Stir in lemon juice and zest. Set bowl over a saucepan of simmering water, ensuring water doesn't touch bowl. Whisk constantly for 8-10 minutes until thickened enough to coat back of spoon. Remove from heat, whisk in cubed butter until melted. Strain through fine-mesh sieve, cover surface directly with plastic wrap, and refrigerate until set, about 1 hour.
07 - Beat butter in large bowl until creamy and smooth. Gradually add sifted powdered sugar, mixing on low until incorporated. Increase speed to medium-high and beat until fluffy, 2-3 minutes. Add lemon juice, zest, and salt. Beat 1 minute longer, adding milk 1 tablespoon at a time until spreadable consistency achieved.
08 - Place first cooled cake layer on serving plate. Spread thin layer of buttercream, pipe a dam around edge, and fill center with half the lemon curd. Repeat with second layer. Top with third cake layer. Frost top and sides with remaining buttercream, creating smooth finish. Decorate with additional lemon zest or fresh lemon slices if desired. Refrigerate 30 minutes before serving to set.