To make the lemon curd, add the lemon juice, large egg yolks, granulated white sugar, salt and lemon zest to a medium saucepan or pot. Whisk everything together.

Heat the lemon mixture over medium-low heat for 20-25 minutes, whisking frequently, until thick. It should coat the back of a spoon once thick enough.

Remove it from the heat and slowly whisk in the cold, cubbed butter.

Once all of the butter is melted in, transfer the curd to a heatproof, small bowl. Place plastic wrap on top of the curd, so it doesn't form a skin, and allow it to cool completely in the fridge.

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x9 inch metal baking pan and line it with parchment paper.

In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Then set aside the flour mixture.

Add the softened butter and granulated white sugar to a large bowl. Cream together for 2 minutes on high speed with an electric mixer. (You can also use the bowl of a stand mixer fit with a paddle attachment.)

Then add in the egg whites, lemon zest and vanilla and mix on medium speed until pale in color and smooth, about 1 minute.

Add the dry ingredients and buttermilk to the wet ingredients a little at a time, mixing on low speed until the cake batter is combined. Scrape the sides of the bowl with a rubber spatula as necessary.

Pour the batter into the prepared baking pan. Bake the cake for 36-42 minutes or until a cake tester comes out clean from the center.

Let the cake cool in the pan for 30 minutes. Then pull up on the parchment paper to remove the cake from the pan. Let it finish cooling on a wire rack until completely cool.

While the cake is cooling, make the lemon syrup by adding the lemon juice and sugar to a small saucepan. Heat the lemon mixture over low heat, whisking frequently, just until the sugar is dissolved into the lemon juice. Remove the lemon syrup from the heat and transfer it to something that you can easily pour from, like a little cream jug.

To make the mascarpone cream frosting, add the heavy cream to a large mixing bowl and mix on high speed with an electric mixer until stiff peaks form, about 1-1 ½ minutes. (If using a stand mixer, use a whisk attachment.)

Add the cold mascarpone cheese to a large, separate bowl and sift in the powdered sugar and add in the lemon zest. Mix on medium speed just until combined and smooth. (If the mascarpone gets "runny" whip on high speed for a bit until fluffy again. I've noticed some brands have more moisture than others.)

Gently fold the whipped cream into the mascarpone mixture with a rubber spatula. Transfer the frosting to a piping bag fit with a decorative tip.

Transfer the cake to a serving plate. Poke holes on the top of the cake using a wooden stick or the handle of a wooden spoon.

Pour the milk over the cake. (Be patient for it to soak in.) Then pour about ¾ of the lemon syrup over the cake, again be patient for it to all soak in. Reserve the rest for serving with each slice.

Transfer the lemon curd to a piping bag fit with a small decorative tip. Pipe lemon curd in the holes of the cake.

Pipe the frosting on to the top of the cake. In between each piping of mascarpone frosting, pipe a little lemon curd. Sprinkle the top of the cake with lemon zest and cut it into 16 squares. Drizzle extra lemon syrup onto each slice of cake when serving and enjoy!