It looks like a totally normal cake....
(And also because it's better to have a surprise birthday cake than a known one.)
And experiment, because I have never attempted a rainbow cake before, and it seemed like an awfully difficult thing to do. And, in fact, it was. I can tell you honestly that this was the most difficult cake I've ever made.
That's not because of the actual cake recipe - I used a pound cake recipe, which is possibly the easiest type of cake to make: a pound of butter, a pound of sugar, a pound of flour, and a few eggs. It's the type of cake my grandmother used to make, and it tastes just like her cake used to taste. It's what my dad calls 'a plain cake - my favorite', and what Nigella ('The Goddess') describes as 'one of those cakes you can never see the point of, until you've bitten into it'.
It makes for a good dense crumb, which you can see in this pic, and I thought it would take the colour layering better than a sloppier mixture.
This picture shows the flour being stirred in and, as you can see, it's a very, very stiff dough, and very hard to stir.
This was the end of the easy bit.
Yes it seems like such an effort, but after you've destroyed several perfectly good cakes by accidentally sticking them permanently to the tin, you'll see the light, and spend as much time on your greasing and lining and flouring process as you do on your actual cake mixture.
Evenly dividing cake mixture into seven parts is more difficult than it sounds. I did it by weighing the entire mixture, dividing the weight by seven and then weighing out that quantity into separate bowls - somehow I managed to mess this up and ended up with one bit that was less than all the others. I then had to mess around plopping teaspoons of mixture from bowl to bowl until I got it approximately right.
here for details), and stuck with the ol' Roy G. Biv mnemonic - red, orange, yellow, green, blue, indigo and violet.
I already had all the gel colours I needed because I have been buying and using them for cake decorating for ages, but if you were going from scratch, it would cost you about $AUD25 to purchase enough colours - you need red, yellow, green, blue and violet and you can make up the rest by combining those.
Of these, the failure was the indigo - I tried for a paler version of violet just by adding less violet colouring to the mixture than I had to the violet mix. But it was sort of grey, and in retrospect I could have got away with just six colours and left the indigo out completely - I doubt anyone would have noticed.
No, this blog is not sponsored, but if you are a GLAD senior exec, you should sling me a few bucks because I'm going to give your product a giant promo.
These Glad sandwich bags are the best invention since sliced bread ... haw haw.
The reason I say this is because you can use them as disposable piping bags by the simple expedient of cutting a tiny bit off one corner. I was planning to do this with the cake - to pipe each layer out - but actually in the end I dolloped it out with a spatula and smoothed it down. In retrospect I should have gone for the piping method with my trusty Glad bags, because dolloping was stressful.
The mixture was so thick that it was a real effort to pull it around with the spatula to create an even layer. Also, even though there was a lot of mixture in total, it had to be spread in very thin layers - about half a centimetre. This was tricky because it was easy to get 'holes' in the layer.
It was especially tricky around the edges because the greased, floured baking paper was SO nonstick that the mixture just wouldn't hold to it at all, and kept coming back up with the spatula!
As the layers built up I developed a bit of a dolloping technique, but I was beset with worry the whole time that my pressure downwards with the spatula whilst spreading the colour layer would distort all the colour layers underneath. This turned out not to be the case - the mixture was firm enough to stay put in its layers pretty well.
And remember - I had no idea what it looked like inside yet. It just looked like a pound cake, with strange indistinct colours underneath the brown outer.
I determined to get my own back on the damn thing.
"Listen here, difficult cake," I snapped (out loud - I really did), "You are not getting ANY MORE of my valuable time. I WAS going to give you fondant icing. Or at least all-over swirled butter icing. But I'm DONE with you. You're getting glace icing or NOTHING."
It took me one minute to mix up some plain glace icing (icing sugar and lemon juice) and pour it over the cake. I made enough that it flowed down all the edges and completely covered the surface. Then I sat down to wait until I could cut it without the icing going everywhere.
During baking as the cake rose, so too did the layers. When I upturned the domed top of the cooked cake to get the bottom as my flat surface, Gravity 'pushed' the centre of the cake upwards from the bottom, giving the 'rainbow curve' effect from the centre.
If I had wanted to have a series of flat layers I could have sliced the domed top of the cake off entirely. At the time, I thought this might take all the violet layer off with it, but the violet, as it was the top layer during the baking process, had risen more than all the others, and was the thickest - so I could have sliced off the dome without losing that colour.
By chance, the glace icing was perfect for it. In taste terms, it was a good topping, because the cake is buttery and moist, and the icing is watery and tangy - each sets off the other. In colour terms it highlighted the red layer beautifully.
Thus ends Dr Cupcake's rainbow cake experiment.
Happy eating of rainbows to everyone, with love from Dr Cupcake!