01 - Preheat oven to 350°F. Grease and line two 8-inch round cake pans with parchment paper.
02 - Whisk flour, baking powder, and salt in a medium bowl.
03 - Beat eggs and sugar with electric mixer on high speed until pale and tripled in volume, approximately 5 to 7 minutes.
04 - Gently fold dry ingredients, lemon zest, and vanilla extract into egg mixture.
05 - Fold in melted butter and milk until just combined. Avoid overmixing.
06 - Divide batter evenly between prepared pans. Bake for 25 to 30 minutes or until toothpick inserted comes out clean.
07 - Let cakes cool in pans for 10 minutes, then turn out onto wire racks to cool completely.
08 - Heat water and sugar in small saucepan over medium heat until sugar dissolves. Remove from heat, cool slightly, then stir in limoncello.
09 - Whip cold heavy cream to soft peaks in chilled bowl. In separate 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 thin layers. Place first layer on serving plate, brush generously with limoncello syrup, and spread with 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.