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 (and that will fit in your oven at the same time). This is because cake batter sat out for a long period of time will affect the final result. If you are halving the recipe to cook it in two batches, it's 275g of butter, sugar and flour, 5 eggs and 2 tsp vanilla extract.
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 until fluffy and creamy, this takes about 3 minutes using an electric mixer
Add half of the eggs and vanilla extract, and whisk until fully incorporated
Add half of the self raising flour and whisk in gently, only 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. Mix until the desired colour is achieved
Divide the mixture between the tins and bake for 15-20 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 buttercream, mix the butter until creamy and paler in colour, around 3-5 minutes. Add the icing sugar, 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
If you want the buttercream to be white, add the Wilton white white and mix in
Start stacking the sponges on your cake board, ideally on a decorating turn table. Spread a layer of buttercream between each sponge, before adding the next one. Make sure to stack them in the rainbow colour order: purple, blue, green, orange, yellow, red
Once the sponges are all stacked and filled with buttercream, cover the whole cake in a "crumb coat" of buttercream. Put it in the fridge for 30 minutes to set the buttercream layer
Remove from the fridge and spread on a final and thicker layer of buttercream and smooth it out using a buttercream smoother/scraper. Use a Put it back in the fridge for 30 minutes
To make the chocolate drip, put the white chocolate and double cream into a bowl and microwave for 20 seconds, then stir. Continue microwaving in 10 second blasts, stirring between each blast, until melted and smooth
I like to add a little bit of the lustre dust to the drip mixture too, but this is optional, it gives a better background colour for the gold 'paint'
Put the chocolate drip mixture into a piping bag or squeezy bottle, if using a piping bag snip a small section off the end
Pipe blobs of drip around the edge of the cake, allowing it to drip down the sides in varying amounts
Once the drips are complete you can cover the whole top of the cake with the chocolate drip mixture it you like. Pipe the remaining mixture all over the top of the cake and use a palette knife to smooth it out. Put the cake in the fridge until the drip is fully set, around 30-60 minutes
When you are ready to paint the fully set drip, mix the lustre dust with the dipping solution in a small bowl to create a 'paint'. Use a cake paint brush to carefully paint the drip gold all over. If the lustre dust dries out, add more dipping solution. Or if you run out of the gold paint, mix up some more with more dipping solution and more lustre dust
Put the remaining buttercream into a piping bag fitted with a nozzle of your choice and pipe some swirls around the edge of the cake
Add sprinkles all over the top of the cake
Leftovers will keep in an airtight container in a cool place for 2-3 days