01 - Preheat oven to 350°F. Grease and line two 8-inch round cake pans with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, and salt until evenly combined.
03 - Beat eggs and sugar with an electric mixer on high speed until pale and tripled in volume, approximately 5 to 7 minutes.
04 - Gently fold the dry mixture, lemon zest, and vanilla extract into the egg mixture until just combined.
05 - Pour in melted butter and milk, folding gently until incorporated. Avoid overmixing to preserve cake texture.
06 - Divide batter evenly between prepared pans. Bake for 25 to 30 minutes until a toothpick inserted in the center comes out clean.
07 - Let cakes cool in pans for 10 minutes, then invert onto wire racks to cool completely before assembly.
08 - Heat water and sugar in a small saucepan over medium heat until sugar dissolves. Remove from heat, cool slightly, then stir in limoncello.
09 - Whip heavy cream to soft peaks in a chilled bowl. In another bowl, whisk mascarpone, powdered sugar, lemon zest, and vanilla until smooth. Fold whipped cream into mascarpone mixture.
10 - Slice each cake horizontally in half for four layers. Place first layer on serving plate, brush with limoncello syrup, and spread mascarpone filling. Repeat with remaining layers, frosting top and sides.
11 - Garnish with lemon slices, zest, white chocolate curls, or edible flowers as desired. Refrigerate for at least 1 hour before serving for optimal flavor and texture.