I own three 8" cake tins so I did three layers of the cake at a time. Only make as much mixture as you have cake tins for as you must not leave cake batter sat for a long period of time as it will affect the final result. To figure out how much you need for each layer, divide the cake ingredients by six. You can also buy disposable foil cake tins if you don't want to invest in more cake tins, but I recommend having at least three of the same size as there are lots of cake recipes on my blog that require three 8" tins!
Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line your 8" cake tins
Make the first three sponges by mixing half of the butter and caster sugar in a large bowl ideally using an electric mixer
Add half of the eggs, milk and lemon zest, and whisk until fully incorporated
Add half of the self raising flour and whisk in until you can't see any flour anymore
Divide the mixture equally between three bowls, add red food colouring to one bowl, orange to another and yellow to the last one. Whisk until the desired colour is achieved
Divide the mixture between the tins and bake for 20 minutes or until a skewer inserted in the middle comes out clean. Remove from the tins and leave them to fully cool on cooling racks
Wash the tins, re-line and grease them, then repeat steps 3 - 7 but add the green, blue and purple food colouring instead
To make the vanilla buttercream mix the butter and icing sugar together. Once it starts to come together add the vanilla extract and milk, and mix until smooth. For best results use an electric mixer. You can add more milk if it is too stiff
Start stacking the sponges on a plate or turn table. Spread a thin layer of buttercream onto each sponge, followed by a generous drizzle of lemon curd, before adding the next one
Once the sponges are all stacked and filled with buttercream and lemon curd, cover the whole cake in a thin crumb coat of buttercream. Put it in the fridge for an hour to firm up
Remove from the fridge and spread on a final and thicker layer of buttercream and smooth it out
Put the remaining buttercream into a piping bag fitted with a nozzle of your choice and pipe some small rosettes around the edge of the cake
Fill the middle of the top of the cake with rainbow sprinkles, and use a brush or your hands to add more sprinkles to the bottom edge of the cake
Leftovers will keep in an airtight container for 3 days