First up lets make the sponge; pre-heat the oven to 180°C/160°C/Gas Mark 4. Grease and line 3x8” round cake tins.
Pop the margarine and sugar in a bowl and cream together using an electric hand whisk or free-standing mixer until light and fluffy. Pour in the eggs, Vanilla Extract and a spoonful of flour, mix together until combined and smooth. Sieve over the flour and fold into the mixture until just combined.
Divide the mixture equally between your prepared cake tins and pop in the oven to bake for 35-40 minutes until golden brown. To test your cakes are baked pop a skewer into the centre of your cake and it should come out clean. Leave to cool in the tins for 15 minutes and then transfer to a wire rack to cool completely.
Pop the butter into a large bowl and beat with an electric hand whisk or free standing mixer fitted with a beater attachment, until smooth and lighter in colour.
Beat in the icing sugar in 4 intervals making sure it is well combined after each addition. Pop in the Vanilla Extract and beat the buttercream until light and fluffy in texture – if it is too firm add 1-2 tsp of milk to soften.
Once your cakes are cooled, use a serrated knife to cut any domed tops off the sponge layers. Pop a sponge layer onto a cake board and spread a layer of buttercream on top, repeat with the other sponge layers to sandwich them together. Cover the top and sides of cake in a thin layer of buttercream to create a crumb coat. Use a palette knife or cake scraper to smooth out the buttercream. Pop your cake in the fridge for 30 minutes to allow the crumb coat to set.
Whilst your cake is chilling, it’s time to colour the buttercream! Pop a couple of spoonfuls of uncoloured buttercream into a piping bag fitted with an open star nozzle and set aside for later.
Divide the remaining buttercream equally into 5 bowls and add 1 tsp of Red Colour Gel to one bowl and mix through until the colour is equally mix through, add some more colour gel to get a stronger colour if needed. Repeat for the other colours until you have a bowl of red, yellow, green, blue and purple buttercream. Pop each coloured buttercream into a piping bag and cut a 1cm hole in the end of each bag.
Once you cake is chilled, pipe 2 rings of purple buttercream around the cake starting at the base of the cake. Repeat with the blue buttercream, then green and yellow working your way up the cake, so you have 2 rings of each colour buttercream on the cake. Finish with the red buttercream piping 1 or 2 rings on top of the yellow buttercream depending how much space you have. Finally spread the remaining red buttercream on top of the cake using a palette knife.
Use a palette knife or cake scrapper to smooth the sides of the cake – it is easiest to do this using a cake turntable. Fill in any gaps with buttercream and continue to smooth the buttercream until your buttercream is smooth. Ta-dah look at that rainbow ombre buttercream!
Pop in the fridge to chill and allow the buttercream to set. Whilst the cake is chilling make the chocolate drip, pop the Chocolate and butter in a bowl and melt in the microwave for 30 seconds then stir. If it is not fully melted pop back in the microwave in 10 second bursts stirring between each burst until you have a lovely smooth ganache.
Take you cake out of the fridge, pop your chocolate ganache into a piping bag and cut a small hole in the end. Pipe drips around the edge of the cake and then fill in the top and smooth out using a palette knife if needed.
Use your reserved uncoloured buttercream in the piping bag to pipe swirls around the top of the cake. Add some Bright and Bold Sprinkles to the buttercream swirls and you’re ready to serve your showstopping rainbow creation!