How to make our rainbow cake: Frequently asked questions

  • By
    Sarah Cook

Sarah Cook provides extra tips for creating the perfect finish to her show-stopping rainbow cake.

Rainbow cake


"A rainbow cake has long been one of my favourites for a special occasion - it looks so impressive, yet it's a relatively simple process. You just need to set aside some time to bake three sets of sponges, then decorate your showstopper - and I've made sure there's plenty of icing so no danger of running short before the end!" Sarah Cook

Sarah's top tips...

Colour testing spongesGet the sponges right

- The sponges are purposely quite dense to make sure they don't rise too much. This means they shouldn't end up uneven, or too rounded, so shouldn't require any trimming. If you need to trim the cake you'll create crumbs, which means it won't look as neat and dramatic. Instead you'll have brightly coloured crumbs in the icing between layers.

- The sponges are also designed to be dense so they come out nice and flat, and obviously when you're layering so many together you want nice and thin sponges so your cake doesn't end up too big to cut. Also, you can't stack six rounded cakes - by the time you get to the top layer the curve will be so great you'll have to snap the top sponge in half to make it fit.

- We've made the sponges in pairs, so you only need two sandwich tins - who has six tins the same size? - not us!

Use the best food colouringTest colourings in water first

- The food colouring must be professional standard and artificial. You won't be able to use natural colours as these fade when baked or heated.

- Pastes are better than liquids as they are often stronger, which means you're adding less extra liquid to the batter. It's the same with weaker, supermarket-standard colourings - the more you have to use, the wetter and denser the sponges will become. If you find yourself adding more than 1 tsp, compensate by adding 1 tbsp more flour for each extra tsp of liquid.

- Some colourings have a funny taste when added in large quantities, so use stronger colouring paste, or double the vanilla extract quantity.

Your frequently asked questions...

Three slices of rainbow cakeHow do I get my frosting the correct consistency?
- We originally made this cake with light cream cheese or mascarpone, so it would be better for you. But, as some of our cooks struggled, we've changed the recipe to full fat cream cheese which is much more stable - but still shouldn't be beaten anywhere near as much as other frosting.

My cake doesn't taste of very much - how can I improve this?
- These are plain vanilla sponges, so should taste like a cross between a plain sandwich sponge and a Madeira cake, but too much colouring may leave a funny taste- see tips above. You can vary the flavour as you wish, as long as you're not adding a great quantity of liquid.

- Add orange or lemon zest, or 1 tsp of almond, raspberry, rose, peppermint, orange or lemon extract in place of the vanilla. Or why not match a flavour to each colour and create a multi-flavoured as well as multi-coloured cake!Zest of an orange

The sponges seem to dense- why is this?
 - If cooked in the right-sized tin at the right temperature, the sponges should have the consistency of Madeira sponge, but they shouldn't be as light as a Victoria sponge - see above for why. 

Why didn't my icing set?
- It was probably beaten too much when making: when cream cheese frosting has been over beaten it becomes too liquid and won't firm up again no matter how long it is chilled. If you use a light hand, follow the instructions exactly as to when and how to add each icing ingredient, the icing should be soft but spreadable as soon as it is made. Then you can chill the cake in the fridge to firm it up a bit if you're transporting or want it firmer. Sponges shouldn't really be kept in the fridge though, so make sure you take it out an hour or so before eating or it will be firm and the flavour will be dulled by the cold.

More useful baking tips to help you achieve success...

Test tube, pestle and mortarBaking is more like science than cooking, so it's super important to treat it as such.

- Make sure you have the right size tin. Tins are always measured across the base or bottom - never the top.

- Make sure your oven is working at the correct temperature - or adjust the dials to accommodate. In the test kitchen we keep an oven thermometer in each oven so we can check it they are always right.

- Don't open the door! Every time you open your oven door the temperature drops by about 30C and takes a good few minutes to get back to the correct temperature. Open too early and your cake won't rise, will be denser, and could even collapse - so try to resist peeking!Measuring spoons

- Don't guess quantities - use digital scales and measuring jugs and spoons. A teaspoon measurement is not the same as the one you use to stir your tea!

Let us know how you get on and feel free to ask any further questions in the comments below...

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