Start by making your lemon curd. In a heatproof bowl, thoroughly whisk all of the ingredients together, apart from the butter. Place over a pan of simmering water, stirring with a wooden spoon until thickened, or it reaches 82°C on a sugar thermometer; this could take up to 25 minutes.
Off the heat, slowly stir in the butter, a cube at a time. Pass through a fine sieve into a container and lay a piece of cling film on the surface. Place the curd in the fridge until needed.
Preheat the oven to 155°C/gas 2 and pop in two baking trays to heat up. Sift the flour, sugar, baking powder and salt into a bowl. In another bowl, whisk the oil, yolks, lemon zest and juice, then mix in the dry ingredients.
Whisk the egg whites and cream of tartar until stiff peaks form, then fold into the batter. Divide between four greased, 20cm, high-sided springform cake tins and place on the hot tray in the oven for 45 minutes, or until golden and a skewer comes out clean.
Make your lemon confit. Peel the lemons with a peeler, then julienne the zest as finely as you can. Blanch it in boiling water, then refresh in cold water; do this three times.
Put the sugar and 200ml of water in a pan and cook over a low heat until the sugar has dissolved, then add the lemon peel and cook until very tender, about 5–7 minutes. Drain and spread out on a lined baking sheet to cool.
Remove the cake from the oven and leave to cool in the tins. Turn the oven down to 75°C/gas ¼.
For the meringue drops, place the egg whites in the clean bowl of an electric mixer and mix on a medium speed. Gradually add the sugar, letting it dissolve before the next addition. Once all incorporated, whisk until you have stiff peaks.
Spoon into a piping bag with a small, round nozzle. Pipe the meringue onto a lined baking sheet in little peaky bulbs. Straight away, scatter the lemon zest over the top and bake for 50 minutes, until the drops are crisp.
Make your buttercream. Whisk the egg whites and sugar in a bowl over a pan of simmering water until the sugar has dissolved and stiff peaks form. Transfer to the bowl of an electric mixer and whisk well. While still warm, gradually add the butter, then the vanilla extract, and whisk until smooth and shiny,
To finish, level each sponge with a knife to make them flat, if needed, then halve each one horizontally, so you have eight thin sponges. Set aside the flattest one for the top.
Put one sponge on a cake stand and spread with curd. Repeat with all but the top sponge and 150g of curd. Add the last sponge and spread a bit of buttercream all over the outside of the cake. Chill for 10–15 minutes.
Keep building up the layers of buttercream two to three more times, chilling in between, until you have a firm, clean finish.
To serve, warm the remaining curd very slightly, until just pourable. Spoon small amounts around the edge of the cake, so it drips down the sides. Arrange meringue drops on the cake, then top with the lemon confit.