Preheat the oven to 190 C / Gas 5. Grease a 20cm cake tin.
Cream together the 225g butter and caster sugar in a bowl until light and fluffy and there are no lumps of butter in sight. Remember to use a bowl large enough to prevent spills.
Gradually beat in the eggs and vanilla extract, being careful not to curdle the mixture. In the unlikely case that this does happen, add a little of the flour.
Fold in the flour using a large wooden spoon, and carefully spoon mixture into separate bowls. The amount of bowls you use is depending on the amount of different colours you want in your cake.
Add selected food colourings to the bowls of cake mixture. (I only have one colourant which is pink, so I ended up making a bright pink mix, a baby pink mix and a normal pink mix. Still worked well).
Spoon the mixtures into the cake tin. Do a teaspoon of one colour, then another colour, then another colour. This gets the multi-coloured 'rainbow' effect.
Bake in the pre-heated oven for around 25 minutes, or untill the whole cake is firm to the touch. You might find that the parts of the cake with the most colourant will take longer to bake than the others.
Remove from oven and leave to cool for around five minutes before taking out of the tin and putting aside on a cool rack.
For icing: Gently cream the 75g butter in a bowl, until smooth and all the lumps of butter are gone.
Gradually mix in the icing sugar, being careful not to slop sugar over the edge of the bowl.
When thoroughly mixed together, add just enough milk to make the icing fluffy and spreadable.
Add food colouring, and spread onto cake.
How to ice a cake video
Watch our How to ice a cake video and see how to ice cakes with ease. You'll be icing perfect birthday and special occasion cakes in no time!