I saw a picture of the famous Rainbow Cake online, and wanted to make it for Small's birthday. Looking up the recipe, I was unhappy to discover that it required something called white cake mix. It really is blindingly white, and I imagine you could do something similar with a classic angel cake mix (egg whites only). I decided instead to use the Hummingbird bakery cupcake batter which is very pale, multiplied by two. I didn't want to make a huge cake as the party was a small group of friends I was taking to the skating rink. But you could multiply by four, enough to create a slim layer for each colour: red, orange, yellow, green, turquoise, blue, indigo, violet.
I use a Magimix, and blend sugar, butter, salt and flour until breadcrumb texture, although you can rub in like pastry with your fingers. Then mix eggs, milk and vanilla, and pour in through the feed tube until you have a smooth batter. Don't overmix.
I bought gel colours red, yellow and blue, and blended them to create the different colours of the rainbow. ie red with yellow for orange, yellow with blue for green, blue with a touch of yellow for turquoise, blue with a touch of red for indigo, and with more red (and using less colour) for violet.
I divided the mixture into eight little bowls and coloured each one. Then I poured the batter carefully into two separate sandwich tins lined with baking parchment. The first four in the one, the second in the other. I baked them at 180C for about 20 minutes until they sprang back when gently pressed in the middle.
Meanwhile, I had melted a bar of white chocolate, and beaten it with icing sugar and unsalted butter until pale and fluffy. When the cake was cold, I sandwiched it with apricot jam, and coated it in the icing. Using a large tube of Smarties, I made a rainbow on top, and finished it with 10 candles, and sparklers in the shape of 1 and 0.