For the Lemon Curd:

1. In a medium saucepan, whisk together lemon juice, egg yolks, sugar, salt, and lemon zest.
2. Heat the mixture over medium-low heat, whisking frequently, until thickened (about 20-25 minutes).
3. Remove from heat and whisk in the cold butter until melted.
4. Transfer the curd to a heatproof bowl, cover with plastic wrap, and refrigerate until completely cool.

For the Lemon Cake:

1. Preheat the oven to 350°F and grease a 9×9 inch baking pan.
2. In a medium bowl, whisk together cake flour, baking powder, baking soda, and salt.
3. In a separate large bowl, cream together softened butter and granulated sugar until light and fluffy.
4. Add egg whites, vanilla extract, and lemon zest to the butter mixture, and beat until smooth.
5. Gradually add the dry ingredients and buttermilk to the wet ingredients, mixing until just combined.
6. Pour the batter into the prepared pan and bake for 36-42 minutes, or until a toothpick inserted into the center comes out clean.
7. Let the cake cool in the pan for 30 minutes, then transfer to a wire rack to cool completely.

For the Lemon Syrup:

1. In a small saucepan, heat lemon juice and sugar over low heat, whisking until the sugar is dissolved.
2. Remove from heat and let cool.

For the Lemon Mascarpone Frosting:

1. In a large mixing bowl, beat heavy cream on high speed until stiff peaks form.
2. In another bowl, mix mascarpone cheese, powdered sugar, and lemon zest until smooth.
3. Gently fold the whipped cream into the mascarpone mixture until well combined.

Assembling the Cake:

1. Poke holes in the top of the cooled cake with a wooden stick or spoon handle.
2. Pour milk over the cake, followed by lemon syrup.
3. Pipe lemon curd into the holes of the cake.
4. Frost the top of the cake with mascarpone frosting, alternating with dollops of lemon curd.
5. Sprinkle with lemon zest and cut into squares before serving. Enjoy!