Begin by making a large batch of cake mix - combine everything in a really big bowl and whisk together until everything is combined and you form a smooth batter.
Equally separate the batter into four different bowls and mix in four different food dyes. Pour into four lined 20cm baking tins and bake for 20/30 minutes, or until a skewer comes out clean. Leave to cool completely on a wire rack before taking off the baking paper and turning upside down - this makes for a flatter cake.
Meanwhile, make your buttercream - whisk in icing sugar (1 cup at a time) to the Stork with Butter until your icing is light and fluffy.
Ice the top of one cake then lay another one on top and repeat with the remaining cakes. With a palette knife cover the sides of the cake (you will have lots of buttercream left - keep this for piping on the mane). Don't worry about completely covering the cake as this is only the initial layer.
Place the cake in the fridge for 30 minutes or so, until the first coat of buttercream has hardened a little, then take it out and apply another coat. Smooth this out with a palette knife, making sure it's even all over the cake.
Make the horn of the unicorn by rolling out a palm sized (100g) piece of fondant on a clean surface covered in icing sugar. Wrap this round a thin stick and spray with gold glitter, then place in the fridge to keep it firm. Roll out a sheet of fondant to roughly 1/2 a centimetre and cut out large ear shapes (see our video for a guide), then smaller ones to fit in the middle. Brush some water on the little inner ears the spray with gold glitter. Stick these to the others with another brush of water (and a tooth pick in between to attach to the cake). Roll out a small amount of black fondant and cut out two half moon shapes to make the eyes.
Stick the horn, ears and eyes on. Split the leftover buttercream between four bowls and add a different food colour to each. Take a little of each coloured buttercream and add to a single large piping bag with a big cake decorating nozzle - this makes an amazing multicoloured effect. Split the leftover colours between different bags for smaller buttercream rosettes.