I remembered seeing somewhere in the universe of the internet a rainbow cake - actually, in the realm of Pinterest (which is scarily addictive - don't click through unless you have too much time on your hands!) there are a huge number of rainbow foods. I'm not a huge fan of excessive food colouring, but sometimes the occasion calls for it, and using gel colours you don't need all that much to get a pretty intense colour. You can get gel colours from most cake decorating stores (like Kiwi Cakes) and a pottle will last ages.
To produce a rainbow of cake, choose a light-coloured batter (I made an orange cake recipe, though I thought it was too dry cooked in thin layers so I won't share the recipe - lemon yoghurt cake would work well). Work out how much batter you need - each layer should be a couple of centimetres thick - and divide it into seven. I made such a large cake (and actually I made a second cake, as there were 150 people at the birthday party) that I made a batch for each layer. Then use gel colours to dye the batter before baking - start with a little and add more until you like the colour. You only need red, yellow and blue colour - red and yellow make orange; blue and yellow make green (though I had green colour so used that) and blue and purple make indigo.
Because the layers are quite thin they cook quickly. The layers for my 30cm cake took about 15 minutes each - the smaller 25cm cake only took about 10. Once cooked and cooled you'll want to trim the tops to make each layer nice and flat. You'll end up with a very colourful bowl of cake scraps. Which you can then feed to your six-month-old nephew, though I can't guarantee they'll impress him:
When the layers are flattish spread a thin layer of icing (anything firm enough to hold - I used a basic buttercream) on each - you don't want too much as it'll detract from the colour, just enough to hold the layers of cake together. I think it looks better with the darker colours at the bottom, so started with violet, building all the way up to red. When they're stacked apply some pressure on top to ensure they're nicely stuck together (sort of smush them down, but not too heavily!) and then ice around the sides, filling in any gaps. I did a thin coat of icing and then covered the cake in these cool (and amazingly easy) roses (you use the Wilton 1M/large open star tip and just pipe swirls from the inside out - I'll write more about this sometime, but I didn't get pictures as by the time I got to this part I was pressed for time!).
I was going for the ultimate deception - see how grown-up it looks? And sort of fits with the whole garden thing for Grandma. The most suspicious thing about the cake was its height - you can see below that it swallowed the whole blade of the knife when cut - those layers do add up!
Grandma was a little confused when we told her she had to cut a wedge - I think she thought we were going to make her eat a huge piece of it! It had the desired effect, though - see her face in the photo on the right! And the whole room gasped with delight when she lifted the slice - audible appreciation definitely massages the ego of the baker, and on the impressing-the-audience scale it possibly even beats my uncle's party trick of eating birthday candles to amuse small children. ;-)
So; a great cake to make to impress; reasonably low effort, just takes a while to bake the layers. Grandma loved it, and had a ball at her party too - many family and friends, and lots of little children to liven things up. It was a stunning day (I guess the weather was trying to live up to Grandpa's party back in February, when it got to 36 degrees!) and it was nice catching up with everyone and gorging on sugar.
What's the best birthday party trick you've seen?