Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk and limoncello. Begin and end with the dry ingredients. Mix until just combined.

Fold in the lemon zest. Divide the batter evenly between the prepared cake pans.

Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

For the frosting, in a mixing bowl, beat the mascarpone cheese until smooth. In another bowl, whip the heavy cream until soft peaks form. Gradually add the powdered sugar, vanilla extract, and limoncello, continuing to whip until stiff peaks form.

Gently fold the whipped cream into the mascarpone until well combined.

Once the cakes are completely cool, place one layer on a serving plate. Spread a layer of frosting on top, then place the second layer on top and frost the top and sides of the cake.

Chill the cake in the refrigerator for at least 1 hour before serving to allow the flavors to meld.