Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line three 8" cake tins that are at least 2" deep
Make the sponge by mixing the butter and caster sugar in a large bowl until fluffy, ideally using an electric mixer
Add the eggs and vanilla extract, and whisk until fully incorporated
Add the self raising flour and whisk in until you can't see any flour anymore
Add your chosen food colouring and whisk until you get the desired shade
Divide the mixture between the tins, use scales for accuracy (mine worked out at about 675g of batter per tin)
Bake them for 35 minutes or until a skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or remove from the tin and place on cooling racks
To make the crumb coat buttercream mix the butter and icing sugar together. Once it starts to come together add the vanilla extract and mix until smooth. I use my electric hand whisk to do this but you can also do it by hand with a spoon. If the buttercream is too stiff, you can add some milk to it
Put about a third of the crumb coat buttercream into a piping bag fitted with a circular nozzle
If the cakes have domed on top, level them off with a cake leveller or a serrated knife
Put one of the sponges on a decorating turntable and use the crumb coat buttercream in the piping bag to pie a border of buttercream around the edge of the sponge - this is to stop the jam seeping into the buttercream that you will spread around the sides of the cake
Fill in the centre of the sponge with half of the jam, then add the next sponge
Do the same again - pipe a buttercream border and fill the centre with the remaining jam, then add the final sponge
Use the rest of the crumb coat buttercream to fully crumb coat the cake, then put it in the fridge for 30 minutes to firm up
Make the pink and blue buttercreams in different bowls. Mix the butter and icing sugar together. Once it starts to come together add the vanilla extract and mix until smooth. I use my electric hand whisk to do this but you can also do it by hand with a spoon. Then add the pink food colouring to one, and blue to the other and mix in well. If the buttercream is too stiff, you can add some milk to it
Put about two thirds of the pink buttercream into a piping bag fitted with a circular nozzle. Do the same with the blue buttercream
To create the striped effect, pipe lines of alternating pink and blue buttercream all around the sides of the cake. Use a cake scraper to smooth the sides and create the stripey effect, making sure to wipe off excess buttercream from the scraper as you go
Once the sides are smoothed to your liking, add more lines of alternating pink and blue buttercream to the top of the cake and smooth it out using a palette knife
Put the cake in the fridge for 30 minutes to firm up
Melt the white chocolate in the microwave, start with a 30 second blast, then stir it, then do 10 second blasts, stirring in between until it's melted
Mix the oil into the melted chocolate
Put the chocolate into a piping bag and snip a small section off the end
Pipe blobs around the edge of the cake, allowing the chocolate to drip down the sides in varying amounts
Use a palette knife to smooth the chocolate around the edge on the top of the cake
Then put the remaining pink and blue buttercream into piping bags fitted with a Wilton 2D nozzle, or other nozzle of your choice, and pipe alternating rosettes of blue and pink buttercream on top of the cake round the edge
Fill the centre with pink and blue sprinkles if desired
Move the cake onto a serving plate or cake stand, and slice for the big reveal! Leftovers will keep in an airtight container in a cool place for 2-3 days