To prepare lemon curd, mix lemon zest and sugar with a fork.
In a large bowl, cream the butter. Add the sugar mixture and beat together. Add the eggs, lemon juice and salt and mix together until thoroughly combined.
Pour the mixture into a pan and cook over low heat while stirring constantly for about 10 to 15 minutes or until the curd starts to thicken and coat the back of a wooden spoon.
Remove from heat. Push the lemon curd through the sieve and into a bowl to make it super smooth. Chill for at least an hour.
To prepare the cake. Preheat oven to 350F. Grease two 5-inch cake pans, and line bottoms with parchment paper.
In a large bowl, sift the flour, baking powder and salt. Set aside.
In a bowl of a standing mixer with a whisk attachment, beat egg whites on medium speed until foamy. Add 2 tablespoons sugar and continue beating until stiff peaks form. Scrape the meringue in another bowl. Set aside.
In the same bowl, mix flour mixture, egg yolks, oil and lemon zest and the remaining sugar until smooth. About 3 minutes.
Fold in half of the meringue into the batter until almost combined. Then fold in the rest of the meringue into the batter until just incorporated. Evenly distribute the batter into the prepared cake pans and spread evenly.
Bake 18 to 20 minutes or until wooden pick inserted in cake comes out clean. Cool in pan on wire rack for about 10 minutes before inverting on rack. Let cool completely. Slice cake layers horizontally to make four cake layers.
To assemble the cake, place one cake layer on a greased baking sheet. Spread with 1/3 cup of the lemon curd. Place another cake layer on top, and gently press down a little bit. Continue until the last cake layer is placed. Refrigerate the cake until you are ready to frost and serve, preferably on the same day.
To prepare frosting, in a large heatproof bowl, combine sugar and egg whites. Using a handheld electric mixer, beat on high speed until foamy for 1 minute. Set bowl over a pan of barely simmering water. Beat on high speed until the mixture is hot to touch.
Remove from heat, stir in vanilla and water, and continue whipping for another 10 minutes until stiff and glossy peaks form.
Smear the meringue frosting thickly on the top and sides of the cake. Using a small metal spatula to create swoops and peaks.
Using a kitchen torch, lightly brown the frosting.
Leave it at room temperature and serve the cake in the same day, as over time the frosting will become grainy.