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

In a mixing bowl, whisk together the all-purpose flour and baking powder until well combined. Set aside.

In a clean bowl, beat the separated egg whites with an electric mixer on medium speed until soft peaks form, about 3–5 minutes.

In a separate large bowl, beat the egg yolks with granulated sugar until pale and creamy, about 4–5 minutes. Stir in lemon zest, lemon juice, and limoncello.

Gently fold the dry ingredients into the egg yolks mixture until just combined. Then gently incorporate the whipped egg whites in batches.

Divide the batter evenly between the two prepared cake pans and bake for 22–25 minutes until golden and a toothpick comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely. Meanwhile, prepare the limoncello syrup.

Make the limoncello syrup by combining limoncello, water, and sugar in a saucepan over medium heat. Simmer until sugar dissolves, about 5 minutes.

Brush the limoncello syrup onto each cake layer while still warm to enhance moisture and flavor.

Beat the mascarpone cheese until smooth, then add heavy cream, powdered sugar, and vanilla extract until fluffy and holding peaks.

Assemble the cake by placing one layer on a plate, spreading mascarpone filling on top, adding the second layer, and finishing with remaining filling.

Cover with plastic wrap and refrigerate for at least 4 hours before serving.