01 - Preheat the oven to 350°F (175°C). Grease and line two 8-inch (20 cm) round cake pans with parchment paper.
02 - In a medium bowl, whisk flour, baking powder, and salt.
03 - In a large bowl, beat eggs and sugar with an electric mixer on high until pale and tripled in volume (about 5–7 minutes).
04 - Gently fold in the dry ingredients, lemon zest, and vanilla extract.
05 - Add melted butter and milk, mixing until just combined. Do not overmix.
06 - Divide batter evenly between prepared pans. Bake for 25–30 minutes, or until a toothpick comes out clean.
07 - Cool cakes in pans for 10 minutes, then turn out onto wire racks to cool completely.
08 - In a small saucepan, heat water and sugar over medium heat until sugar dissolves. Remove from heat, cool slightly, then stir in limoncello.
09 - In a chilled bowl, whip heavy cream to soft peaks. In another bowl, whisk mascarpone, powdered sugar, lemon zest, and vanilla until smooth. Gently fold whipped cream into mascarpone mixture.
10 - Slice each cake in half horizontally for four thin layers (optional). Place the first layer on a serving plate. Brush generously with limoncello syrup. Spread a layer of mascarpone filling over the cake. Repeat with remaining layers, finishing with mascarpone frosting on top and sides.
11 - Decorate with lemon slices, zest, white chocolate curls, or edible flowers as desired.
12 - Chill for at least 1 hour before serving for best flavor and texture.