Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans and line with parchment paper.

In a mixing bowl, whisk together 1.75 cups of all-purpose flour and 2 teaspoons of baking powder.

Beat 4 egg whites until soft peaks form, about 3-5 minutes.

In another bowl, combine 4 egg yolks and 0.75 cups of granulated sugar, beating until pale and thick. Gently mix in lemon zest, lemon juice, and limoncello.

Fold the dry ingredients into the wet ingredients until just combined, then gently incorporate the whipped egg whites.

Divide the batter between the prepared pans and bake for 22-25 minutes, until golden brown. Allow to cool for 10 minutes before transferring to a wire rack.

For the limoncello syrup, combine 0.25 cups of limoncello, 0.25 cups of water, and 2 tablespoons of sugar in a saucepan. Heat until dissolved.

Beat 8 oz of mascarpone cheese until smooth, then mix in 1 cup of heavy cream, 0.5 cups of powdered sugar, and 1 teaspoon of vanilla extract until fluffy.

Brush the cooled cake layers with limoncello syrup and frost between the layers and on the top and sides.

Refrigerate the cake for at least 4 hours before serving to meld flavors.