Preheat the oven to 170c/325f. Grease and line five 18cm diameter round baking tins with baking parchment (if you don't have five, you can do two or three cakes at a time instead, just make sure you allow the first batch of cakes to cool for 15-20 mins before removing from the tin).
Beat the butter and sugar together in a large bowl until light and fluffy. Beat in the eggs and vanilla extract.
Sift in the flour and baking powder, then mix until combined.
Divide the mixture between five bowls (I find it's best to use scales for accuracy).
Mix a half a tsp of each of the gel colourings into each bowl. Use more or less depending on how vibrant you want the cake colours to be.
Spoon each of the bowls of cake batter into separate cake tins and bake in the oven for 20 minutes.
Take out of the oven and leave to cool for 15 mins, then remove from the tins and place on a cake rack until completely cool.
Now make the buttercream. Beat the butter, confectioners's sugar and vanilla extract together until smooth and creamy. Beat in the Vimto cordial, then add the milk, one tablespoon at a time, whilst mixing until you get a soft buttercream that you can spread. Mix in 1/4 tsp of red food colouring (a little more if you want it really red).
Once cool, trim the tops off the cakes using a sharp knife so the top is completely flat.
Place one of the cakes on a plate or cake stand. Top with 2 heaped tbsp of the buttercream and carefully spread to cover the top of the cake. Place another one of the cakes on top and repeat until all of the layers are piled up, with buttercream in between them.
Spread half of the remaining buttercream on the top and sides of the cake. It's best to add lots of little blobs and join them up - this helps to stop lots of crumbs getting in the buttercream.
Place the cake in the fridge for 1 hour to chill, then remove from the fridge and spread on the remaining buttercream (you may need to beat the buttercream again with a splash of milk to loosen it up again).
Remove the rainbow belts from the packets and trim them so the length of each rainbow belt is equal to the height of the cake.
Arrange the rainbow belts all around the cake and place the cake back in the fridge for 30 minutes to chill.
Remove the cake from the fridge and place the red nose centrepiece on top, in the centre. Arrange the Maltersers on top of the cake. Push them down a bit so they stick to the buttercream.
Use the white writing icing to make 2 little dots on some or all of the Maltesers for the whites of the eyes. Add a smaller dot of black or brown icing for the pupils. Make the red noses using the red writing icing.
Serve straight away, or cover and store the cake at room temperature for up to a day before slicing and serving.