Last year, I made this cake for my birthday, and it was a hit with my friends. However, I was never quite happy with the buttercream that I used with it – it was too sweet and made eating this cake quite a challenge.  So, I decided to give it another shot and take it in to share with my co-workers. So, being that I work at a design agency, I think it went over quite well in ‘yeah, we work with colour’ kind of way.

The key to this cake is time. And a set of scales. Be sure to watch the layers when they’re in the oven, because they’re really thin and you can easily overbake them. Try different colour combinations as well! It would be fun to see one that’s more of a monochromatic theme.

For this recipe, I used the batter below, but you can cheat and use a couple of boxed cake mixes instead. For the buttercream this time around, I made three batches of the lemon buttercream that I used on my Earl Grey Cupcakes. I made two of the batches with the lemon zest and used them to coat in between the layers and to do the crumb coat. The third batch for the final coating didn’t have the lemon zest in order to have a pure white finish with no lemony bits.

Ingredients

• 1 cup (230 grams) butter, room temperature
• 2 1/3 cups sugar
• 5 egg whites, room temperature
• 2 teaspoons vanilla
• 3 cups flour
• 4 teaspoons baking powder
• 1/2 teaspoon salt
• 1 1/2 cups (355 milliliters) milk, room temperature
• gel food colouring – I used the Wilton food colours.

Directions

1. Preheat oven to 350° F (approx 175°C). Butter your 8-inch cake tins and line with baking paper. I only have two pans, so I had to do this in multiple stages, just clean and reuse them between each layer – like I said earlier, this takes time! Then weigh the empty large bowl that you will be mixing everything in and take note of it. (All you Americans back home, this will be easier in the long run if you set your scale to grams.)

2. In a medium bowl, sift together the flour, salt, and baking powder. Separate your egg whites into a separate small bowl and set aside.

3. In the large bowl, cream the butter and sugar. Then slowly add your egg whites, mixing in as you go, followed by the vanilla.

4. Add in the milk and the dry ingredients, alternating between the two until you have combined everything completely.  Then weigh your filled mixing bowl. Subtract the weight of your empty bowl to get the weight of the batter you need to divide. Divide that number by the number of layers you intend to make, so if you’re doing the complete rainbow, it will be six.

5. As with the mixing bowl, weigh the bowls you will use to colour the layers while empty (or just set them on the scale and tare it). Gradually add in your batter until it weighs its even portion. Repeat this process with each of the bowls until you have enough for all of your layers.

6. Mix in the gel food colouring to each of your batters until they reach your desired hue. Pour into your cake tins, using a spatula to get as much of the batter out as possible. The layers will be very thin!  Bake for about 15 minutes, or until a cake tester comes out clean.

7. Let the cakes cool in the pan for about five minutes before transferring to a wire rack to cool completely. While all of the layers are cooling, make the first two batches of buttercream (the ones with the lemon zest).

8. To stack the layers, coat each one with a thin layer of the icing, trying to keep them as even as possible to prevent toppling over. Once all of the layers are stacked, coat the top and outside with the rest of the icing, creating a crumb coat. Place the cake in the fridge to allow the crumb coat to set.

9. While the cake is in the fridge, mix up the third batch of buttercream (the one without the lemon zest). Use this batch to coat and complete the cake with a clean, white finish!