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

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

3. In a large bowl, beat the egg yolks with half of the sugar until pale, thick, and ribbon-like.

4. Beat in the melted butter, limoncello, lemon juice, lemon zest, and vanilla until smooth.

5. Gently fold the dry ingredients into the yolk mixture just until combined.

6. In a clean bowl, beat the egg whites until foamy, then gradually add the remaining sugar and beat until soft, glossy peaks form.

7. Fold about a third of the egg whites into the batter to lighten it, then gently fold in the rest in two additions.

8. Pour the batter into the prepared pan and smooth the top.

9. Bake for 28–32 minutes, or until the top springs back and a toothpick inserted in the center comes out clean.

10. Cool in the pan for 10 minutes, then remove the springform ring and cool completely on a wire rack.

11. For the syrup, warm the limoncello, sugar, and water in a small saucepan over low heat, stirring just until the sugar dissolves. Do not boil.

12. Brush the cooled syrup generously over the cooled cake.

13. For the filling, beat the mascarpone with powdered sugar, limoncello, lemon zest, and vanilla until smooth, being careful not to overbeat.

14. In a separate bowl, whip the cold heavy cream to stiff peaks, then gently fold it into the mascarpone mixture.

15. Slice the cake horizontally into two even layers using a long serrated knife.

16. Spread half of the mascarpone filling over the bottom layer, replace the top layer, then spread the remaining filling over the top and sides.

17. Refrigerate the assembled cake for at least 1 hour before slicing and serving.