Pre-heat the oven the 180°C/160°C/Gas mark 4. Grease and line a 8” round cake tin. Place the margarine and sugar into a large bowl and cream together until light and fluffy.
Add the eggs and Vanilla Extract and beat together to form a smooth mixture – if the mixture begins to curdle add a spoonful of flour. Sift the flour on top and fold into the mixture until just combined.
Pour the mixture into the prepared cake tin and smooth the top. Bake in the oven for 30 – 35 minutes until lightly golden and when a skewer is inserted into the centre of cake it comes out clean. Leave the cake to cool in the tin for 15 minutes, then remove, and place on a cooling rack to cool completely.
Once the cake has completely cooled, cut in half to create the rainbow shape. Spread a layer of buttercream onto one half of the sponge and then sandwich together with the other sponge.
Stand the cake on our cake board or serving plate so the cake looks like a rainbow. Cover the top and sides of the rainbow in a thin layer of buttercream.
Stick the smarties onto the front of the cake in a rainbow pattern.
With the remaining buttercream place in a piping bag a cut off the end to create a 1 cm hole. Pipe blobs of buttercream around the base of the rainbow to resemble clouds.
Your rainbow cake is now ready to enjoy!