It sounds really easy when you first think about it but there's actually a lot of maths and organisation needed to make it all work. Luckily, I've done all the hard work so you'll have no problem in making and assembling it yourself.
Be prepared to use every bowl in the house and I'd suggest that you use a digital weighing scales and beg, borrow or steal four 8-inch loose bottomed cake tins because this will make your life so much easier. I only had two, so it was like the neverending Bakeoff in my house. Oh, and a can of Cake Release spray makes the sponges pop out of the tin with one swift tap - no need to cut out 7 baking parchment circles. Another thing you need to know is that the total cake mix weighs 1400g so that's 7 bowls with 200g of mixture in each (this is where the digital scales are very useful as you can put the bowl on the scales, set the weight to zero and just spoon in 200g of mixture)
Alicia's Rainbow Birthday Cake
350g Butter, very soft
350g Caster Sugar
6 Eggs, at room temperature
350g Self-Raising Flour
1 tbsp Vanilla Extract
2-3 tbsp Milk
Preheat your oven to 170c/Gas Mark 4 and spray your cake tins with some Cake Release.
In a very large bowl, cream the butter and sugar together until light and fluffy. Add the eggs, with a tablespoon of flour, one at a time (this stops the mixture from curdling) and the vanilla extract
Add the rest of the flour and then thin down the batter with the milk, a tablespoon at a time until it's a loose dropping consistency
Time to split the mixture into 7 bowls and start the colouring and flavouring process. I found that Wilton gel food colours gave the best depth of colour, Sugarflair are also very good. I used a mixture of Dr. Oetker and Steenbergs flavourings and the results were fantastic.
As you can see, I photographed them in order : red (strawberry), yellow (lemon), pink (vanilla), green (mint), purple (vanilla), orange (orange) and blue (vanilla).
Spoon the mixture into the prepared tin - this is where you realise that having four cake tins makes a lot of sense - and bake for 12 minutes. Cool for 2-3 minutes in the tin and turn out onto a wire baking rack. Keep going until all seven layers have been baked.
And now it's icing time. Alicia loved this and asked me if I made extra so she could eat it with a spoon! It's a very basic vanilla cream cheese icing and doesn't overpower the cake layers.
Vanilla Cream Cheese Icing
100g Butter, very very very soft
250 Cream Cheese, at room temperature & full fat
600g Icing Sugar
Put all the ingredients into a large bowl and beat well for about 6-7 minutes. The longer you beat this, the lighter and fluffier it becomes...
It's cake assembly time. Place a few dabs of the icing on the plate and put the base layer on top - this acts as glue and stops the cake sliding around the place. Spread 2-3 tbsp of the icing over the first sponge, top with the next and repeat until you're putting the seventh sponge on top.
Apply a thin layer of icing to the top and sides of the cake (this is called the crumb coat) and stick the cake in to the fridge for at least 30 minutes to let the icing firm up. You might need to move shelves to accommodate the cake as it is rather tall at this stage. Take the cake out and finish icing it, making it as smooth and pretty as possible. Decorate with pretty things. I used Dr. Oetker Sugar Butterflies as the birthday girl likes the colours, but don't go too mad because you want the focus to be on what's inside...