Preheat your oven to 160ºC/140ºC fan and line three 8"/20cm springform tins with parchment paper and leave to the side for now.
Weigh 5-6 medium eggs in their shells and get as close to 300g as possible (e.g. 312g)
Then add 312g of butter and caster sugar to a bowl and beat until light and fluffy.
Add in the eggs, and 312g of self raising flour and beat again until smooth.
Split the mixture evenly into three bowls, and add three of the colours each (for example, red, yellow and orange.)
Bake the cakes in the oven for 30 minutes (or until baked - test with a skewer).
Once baked, leave the cakes to cool in the tin for 10 minutes before removing to a wire rack to cool fully.
Repeat this process again and use the other three colours (for example, green, blue and purple!). Once all the cakes are baked, leave them all to cool fully.
(You can wrap the sponges in clingfilm once they have cooled and freeze over night to make decoratin easier but this is optional!)
Buttercream
Once all of your cakes are cooled, you can make the buttercream.
Beat the room temperature unsalted butter on its own for a few minutes to soften it and make it supple.
Gradually add the icing sugar beating very well each time until its all mixed in.
Keep beating and add the white food colouring and vanilla extract and beat for a few minutes.
Buttercream Decoration
Using a 10" cake board, add your first sponge. I start with purple as its the bottom colour.
Add a couple tablespoons of the buttercream on to the sponge and smooth over. Repeat this with blue sponge, then green, yellow, orange and finally red at the top.
Using a small amount of buttercream, smooth over the sides of the cakes and fill in any gaps and create a crumb coat.
Refrigerate or freeze the cake on the board for an hour or so to chill and firm to make the next stage of decorating easier.
Once chilled, add on lots more buttercream around the sides of the cake with a small angled spatula, and smooth over using a large metal scraper until the sides of the cake are smooth like in the photos!
Carefully press some of the sprinkles into the bottom of the cake using the palm of your hand to create the sprinkle effect!
Once this has been done, refrigerate again whilst you make the drip!
Decoration
Add the white chocolate and double cream to a bowl and heat in a microwave in short 10 second bursts stiring well each time.
Once melted and combined, add the pink food colouring and mix until smooth!
Add the pink ganache to a piping bag and carefully cut off the end and drip down the sides of the cake. The more you push over the side of the cake with the piping bag, the further it will drip!
Add the leftover buttercream to a piping bag with your favourite piping tip and pipe swirls of buttercream onto the top of the cake (like I have done in the photos)
Finally sprinkle on some etra sprinkles and leave the cake to set in the fridge fully.
In the hot weather I store the cake in the fridge until it has been portioned.
Notes
This cake lasts 4-5+ days once made!
The colours that you use and what order are up to you, this recipe is just how I did it! You can swap up the order or whatever you prefer!
You should avoid supermarket colours as they are not strong enough.