Preheat the Oven: Preheat your oven to 350°F (180°C). Grease and line two 6-inch cake pans with parchment paper.
Prepare the Wet Ingredients: In a large jug, whisk together the egg whites, whole egg, milk, and vanilla extract. Set aside.
Mix the Dry Ingredients: In the bowl of an electric mixer fitted with the paddle attachment, combine the cake flour, sugar, baking powder, and salt. Mix on low speed for 1 minute. (Alternatively, you can do this by hand.)
Incorporate the Butter: Add the cold, cubed butter to the dry ingredients. Mix on low speed until the mixture resembles a fine, crumbly texture.
Combine Wet and Dry Mixtures: Gradually add the wet mixture into the dry ingredients in two separate batches. Scrape down the sides and bottom of the bowl before and after each addition. Mix until the batter is light, fluffy, and well incorporated. Give the batter a final fold to ensure everything is fully mixed.
Bake the Cake: Divide the batter evenly between the prepared cake pans. Bake for 25-35 minutes, or until a cake tester inserted into the center comes out with a few crumbs. Be careful not to over-bake—check the cake at 25 minutes and adjust the timing as needed.
Cool the Cake: Let the cakes cool in the pans for about 20 minutes before transferring them to cooling racks. Once cooled, slice each cake in half horizontally to create four layers.
Assemble the Cake: Generously spread lemon curd between each layer of cake.
Make the Meringue Icing: Place the egg whites and sugar into a heatproof bowl. Whisk to combine, then place the bowl over a double boiler (bain-marie). Whisk constantly, heating the mixture until the sugar dissolves and it feels warm to the touch.
Whisk the Meringue: Remove the bowl from the heat and transfer it to a stand mixer. Using the whisk attachment, beat the mixture on high speed until medium-stiff glossy peaks form. Add the pinch of salt and vanilla extract, then mix to combine.
Decorate the Cake: Spread the meringue icing generously over the top and sides of the cake. Use a kitchen torch to gently toast the meringue until golden brown.
Storage: Store the cake at room temperature for up to 3 days.