Heat oven to 350oF (180oC). Butter and line three 6-inch cake pans with parchment paper.
In a large jug combine and whisk together the egg whites, whole egg, milk, and the vanilla extract. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the dry ingredients together on low speed for 1 minute. (You can also do it by hand)
Add the cold butter and continue to mix on low until the mixture is a fine crumbly texture.
Add the liquid mixture into your dry ingredients in 2 separate batches making sure to scrape the sides and bottom before and after each batch, mixing until light, fluffy, homogenous and well incorporated. Fold once or twice to ensure the batter at bottom of bowl is incorporated.
Divide the batter evenly into the prepared pans.
Bake until a cake tester comes out with a few crumbs when inserted into the center, about 25-35 minutes. Be very careful to not over-bake. Check cake at 25 minutes to see how it is doing and judge the timing from there.
Once baked, let the cake cool on racks for 20 minutes before removing them from the tins.
Slice the cakes in 2 to create 4 layers. Generously spread lemon curd between the layers of the cake.
For the Meringue icing: Place the whites and sugar in the bowl. Whisk to combine.
Place the bowl over a ban marie. Whisk constantly, heat the egg white mixture until the sugar dissolves and it is warm to the touch.
Once warm, transfer the bowl back to the mixer. With the whisk attachment, beat the egg whites on high until medium-stiff glossy peaks. Add in the salt and vanilla and mix until combined. Use immediately.
Spread the meringue frosting all over the cake. Take a torch and toast the outside of the cake.
Store the cake at room temperature for up to 3 days.