Preheat the oven to 350°F (175°C). Grease and line a 9-inch round cake pan with parchment paper.

In a mixing bowl, whisk together flour, baking powder, and salt.

In another bowl, cream the softened butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition.

Stir in limoncello and vanilla extract. Gradually add the dry ingredients to the wet mixture, alternating with milk, and mix until just combined.

Pour the batter into the prepared cake pan and bake for 25 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a mixing bowl, combine mascarpone cheese, heavy cream, powdered sugar, limoncello, and lemon zest.

Using an electric mixer, beat the mixture until it forms soft peaks.

In a small saucepan, combine water, sugar, limoncello, and lemon juice. Bring to a boil, then reduce heat and simmer until the sugar is dissolved.

Once the cake is cool, slice it in half horizontally. Brush the cut sides with lemon syrup.

Spread half of the mascarpone filling on the bottom layer, then place the top layer back on and spread the remaining filling on top.

Chill the cake in the refrigerator for at least 1 hour before serving.