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 well blended.
03 - Beat eggs and sugar with an electric mixer on high speed until pale and tripled in volume, approximately 5–7 minutes.
04 - Gently fold the flour mixture into the egg mixture along with lemon zest and vanilla extract until just incorporated.
05 - Fold in melted butter and milk until combined, taking care not to overmix the batter.
06 - Divide batter evenly between prepared pans. Bake for 25–30 minutes or until a toothpick inserted into the center comes out clean.
07 - Allow cakes to cool in pans for 10 minutes, then invert onto wire racks to cool completely before assembly.
08 - Combine water and sugar in a small saucepan over medium heat. Stir until sugar dissolves completely. Remove from heat, cool slightly, then stir in limoncello.
09 - Whip cold heavy cream to soft peaks in a chilled bowl. In a separate bowl, whisk mascarpone, powdered sugar, lemon zest, and vanilla until smooth. Gently fold whipped cream into mascarpone mixture.
10 - Slice each cake layer horizontally in half for four thin layers. Place first layer on serving plate, brush generously with limoncello syrup, and spread with mascarpone filling. Repeat with remaining layers, finishing with frosting on 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 to allow flavors to meld.