Pre-heat your oven to 180°C/ 365F and line the bottom of your baking tins with baking paper.
In a large bowl, combine the milk and the apple cider vinegar. Put this to the side and allow it to curdle for a couple of minutes. Then add the oil and sugar. Mix it in until combined. In another bowl, sieve the flour, salt, baking powder, soda + salt and combine with a wooden spoon. Using a large spoon, add the dry mixture to the wet, whisking as you go.
Pour your cake batter into two 6 inch tins, evenly (the exact ones that I use are linked in the text above this recipe). Bake for 45 minutes, or until a knife comes out of the centre clean. Leave the cakes to cool completely.
Once your cake is completely cool, slice the cake into four even layers. Before I do this, I cut the top off of the cake (which would have grown above the tin). Use a sharp knife to cut the top of the cake off whilst it is still in the cake tin. Use the edge of the cake tin to guide you, making this a super even cut. Once the top is off, I use a cake leveller to cut the four sponge layers. Using a leveller is WAY easier than using a knife. You can buy one here https://amzn.to/3bwXZdd (or via the link in the text above the ingredients). To use this, set your leveller to the height you want your cake to be, and then use it to cut your sponges.
In a standing mixer, mix the butter for around 2 minutes, until creamed. Add the salt and extract and mix again for another 30 seconds. Then, add the icing sugar and mix on a low speed for around 30 seconds and then increase to a high speed, until combined and fluffy (around 4-5 minutes). If you think your icing is too soft you can always add more icing sugar. Equally, if you think your icing is too thick, add a tablespoon of water.
Using a cake plate, layer all of the sponges up evenly with icing. Within each icing layer, create an icing barrier around the edge and place approx 1 tablespoon of lemon curd in the middle. Piping bags make this step easier.
Using the piping bag with icing in, spread the icing around the edge and the top of the cake. Use a cake spatula to smooth it all out. If you don’t have these, you can do more of a rustic pattern with a knife. Place the cake in the fridge to cool for as long as possible after this step (ideally at least a couple of hours).
Decorate the top of the cake in whatever way you want. I did icing swirls and then placed lemon and lemon biscuits on top. ENJOY!