For the Lemon Curd:

1. In a medium saucepan, whisk together the lemon juice, egg yolks, granulated sugar, salt, and lemon zest.
2. Heat the mixture over medium-low heat for 20-25 minutes, whisking frequently, until thickened.
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 chill in the fridge until cold.

For the Lemon Cake:

1. Preheat your oven to 350°F (175°C) and prepare a 9×9 inch baking pan with parchment paper.
2. In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt.
3. In a large bowl, cream together the butter and granulated sugar until light and fluffy.
4. Mix in the egg whites, vanilla extract, and lemon zest until smooth.
5. Gradually add the dry ingredients and buttermilk to the wet ingredients, mixing until 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 before transferring to a wire rack to cool completely.

For the Lemon Syrup:

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

For the Lemon Mascarpone Frosting:

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

Assembling the Cake:

1. Poke holes in the cooled cake and pour over the milk and lemon syrup.
2. Pipe lemon curd into the holes, then frost the top of the cake with the mascarpone frosting.
3. Sprinkle with lemon zest and slice into squares. Drizzle with extra lemon syrup before serving. Enjoy!