Preheat the oven to 180°C/ 160°C fan oven/ Gas Mark 4. Grease and line 5 x 7” Cake tins – if you do not have enough cake tins bake the sponges in batches.
In a bowl cream together the margarine and sugar using a free standing or hand held mixer.
Add the eggs and Vanilla Extract and mix until smooth – if the mixer begins to curdle add a spoonful of flour.
Sieve the flour on top and fold into the mixture until all combined.
Divide the mixture between 5 bowls and add a different Colour Gel to each bowl and mix until each colour is evenly mixed through the batter.
Put a different coloured batter into each prepared cake tin and place in the oven. Bake the cakes for 20 -25 minutes until risen and a skewer inserted into the centre comes out clean. Once baked leave to cool in the tins for 10 minutes and then remove and place on a cooling rack to cool completely.
Whilst the cakes are cooling make the buttercream. Place the butter in a bowl and beat with a freestanding or hand held mixer until smooth and glossy. Add the icing sugar in four stages making sure the icing sugar is mixed into the butter before the next addition.
Add the milk and whisk the buttercream until it is light and fluffy – you might not need to add all the milk add 1 tbsp at a time until the desired texture for the buttercream is achieved.
Divide the buttercream into 6 bowls. Leave one bowl white and colour the other 5 bowls with a different coloured buttercream. Stir the colour through each bowl of buttercream until colour is evenly mixed through. – you want pastel shades of colour rather than bright colours.
Level each layer of sponge, using a sharp knife to cut off any domes on top of the sponge.
Using a round cutter about 8cm in diameter cut a hole in the centre of the blue, green and yellow sponge. – this is to create the piñata inside of the cake.
Place a small blob of buttercream onto your cake board/ serving plate and place your purple sponge layer on top.
Place a spoonful of white buttercream on top of the purple sponge and using a palette knife spread the buttercream evenly over the sponge.
Place the blue sponge on top and cover with buttercream, repeat with the green and then yellow sponge.
Pour the Unicorn Confetti Sprinkles to fill the hole in the centre of the cake. Leave a few spare to sprinkle on top of the cake once you’ve finished decorating.
Sandwich the pink sponge layer on top of the cake and crumb coat with the remaining white buttercream. Place the cake in the fridge to set for 30 minutes.
Once your cake has chilled remove from the fridge. Starting with the purple buttercream palette a band of buttercream around the bottom of the cake – don worry about smoothing out the buttercream at this stage. Palette a band of blue buttercream above the purple, then green and yellow. Cover the top of the cake with pink buttercream and the remaining edge towards the top of the cake.
Using a cake scraper smooth the sides of the cake so the buttercream colours merge together – not too much as you want the defined rainbow colours.
Place the left over coloured buttercream into piping bags fitted with a star nozzles or no nozzle of you don’t have enough – you can cut the end of the piping bag and create buttercream kisses. Pipe buttercream swirls in alternating colours around the top of the cake.
Finish the cake by scattering with Unicorn Confetti Sprinkles and your ultimate rainbow cake is ready to enjoy!