Preheat your oven to 350°F (175°C) and prepare two 8-inch round cake pans by greasing and lining them with parchment paper.

In a large bowl, whisk together the all-purpose flour and baking powder until combined and set aside.

In a separate bowl, beat the egg whites until soft peaks form, about 3-5 minutes, then set aside.

In another bowl, beat the egg yolks and granulated sugar until pale and creamy, then mix in lemon zest, lemon juice, and limoncello.

Gently fold the flour mixture into the egg yolk mixture, then fold in the egg whites until no streaks remain.

Divide the batter into prepared pans and bake for 22-25 minutes, until golden and a toothpick comes out clean. Allow to cool.

To make the limoncello syrup, combine limoncello, water, and sugar in a saucepan, stirring until dissolved, and simmer for 5 minutes.

Once cooled, brush the limoncello syrup generously over the tops of both cake layers.

For the frosting, beat mascarpone cheese until smooth, then add heavy cream, powdered sugar, and vanilla, beating until thickened.

With one cake layer on a serving plate, spread an even layer of mascarpone frosting on top, then place the second layer and frost the entire cake.

Refrigerate the cake for at least 4 hours or overnight before serving for the best flavor.