Preheat the oven to 180°C / 160°C fan
Grease and line 3 20cm cake tins
Prepare the buttermilk: weigh out 1 tbsp lemon juice in a bowl and top up with almond milk until total weight is 250ml. Stir and leave to sit for 5 minutes.
Sieve flour into the bowl of an electric mixer. Add sugar, baking powder, xanthan gum and salt.
Add the eggs, homemade buttermilk, vegetable oil, lemon extract, vanilla extract and water
Beat on a medium speed for 2 minutes
Distribute the batter evenly between 3 20cm round tins
Bake each for 25 minutes or until a skewer comes out clean
Cool cake in the pan for 5 minutes before transferring to a cooling rack. Wait at least 1 hour before assembling.
Level each cake layer by using a large knife or cake leveller to carefully slice a thin layer off the top, creating a flat surface
Place the first cake layer onto a cake board or plate
Spoon some icing into a piping bag attached to a large round nozzle
Pipe an even layer of icing onto the first cake layer, in a spiral shape. Spread out evenly with a knife or offset spatula.
Pipe a ring of icing around the outer edge of the cake, on top of the first layer of icing. This will act as a bank to support the lemon curd.
Spoon a layer of lemon curd over the first layer of icing, spreading up to the edges of the 'bank'
Place the second cake layer on top, checking everything looks straight. Repeat the steps above to add the icing/lemon curd. Place the final cake layer on top.
Spread icing evenly over the outside of the cake. This of course doesn't have to be perfect, but if you want to create an extra smooth finish, this can be achieved using a cake scraper. An initial thin crumb coat of icing is spread over before refrigerating the cake. This is followed with a second thicker layer of icing which is scraped until smooth. There's a great how-to video linked above if you fancy giving it a go!
Decorate as desired! I added some extra lemon slices and a sprinkling of zest.