01 - Preheat the oven to 350°F. Grease and line two 8-inch round cake pans with parchment paper.
02 - In a medium bowl, whisk 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 5-7 minutes.
04 - Gently fold dry ingredients, lemon zest, and vanilla extract into the egg mixture until just incorporated.
05 - Pour melted butter and milk into the batter, folding gently until combined. Take care not to overmix.
06 - Divide batter evenly between prepared pans. Bake for 25-30 minutes until a toothpick inserted into the center comes out clean.
07 - Let cakes cool in pans for 10 minutes, then turn out onto wire racks to cool completely before assembly.
08 - Heat water and sugar in a small saucepan over medium heat 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 another bowl, whisk mascarpone, powdered sugar, lemon zest, and vanilla until smooth. Gently fold whipped cream into the mascarpone mixture.
10 - Slice each cake horizontally for four thin layers if desired. 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 preferred.
12 - Refrigerate the assembled cake for at least 1 hour to allow flavors to develop and texture to set.