All Natural Rainbow Cake

For the cake:

1. Prepare the 6" round cake pans by lining each pan with parchment paper.
2. Preheat oven to 325°F/163°C.
3. Separate the egg whites from the egg yolks into 2 different bowls, while cold from the fridge.
4. Combine the egg yolks with 60g sugar and whisk together.
5. Add in the vegetable oil, milk, vanilla extract and whisk again.
6. Sift in the cake flour over top of the egg yolk mixture and fold gently to combine, until there are no lumps.
7. Weigh out the egg yolk batter.
8. Divide the egg yolk batter evenly into 6 different bowls (about 66g each).
9. Designate and add one colour to each bowl.
10. Mix the colour into the egg yolk batter evenly with a spoon or spatula. Set aside.
11. Add the egg whites to a stand mixer bowl fitted with whisk attachment, and whip until foamy.
12. Add in cream of tartar (or lemon juice) and continue to whip.
13. Once full of bubbles, gradually add in granulated sugar and increase mixer speed.
14. Whip until the egg white meringue reaches stiff peaks.
15. Weigh whipped the egg whites.
16. Divide the egg whites evenly into 6 different coloured egg yolk batter (about 45g each).
17. Fold the egg whites into the egg yolk batter, incorporating the coloured powder evenly.
18. Repeat with each colour.
19. Transfer one coloured cake batter into a cake pan, and repeat with however many pans you have.
20. Give the cake pan a few taps to release any trapped air bubbles.
21. Bake the cakes at 325°F/163°C for 14-15 minutes, or until the surface springs back when pressed lightly.
22. Remove the cakes from the oven, give the pans a bang on the counter, and let cake cool in the pans for 5 minutes.
23. Transfer the cakes to a wire rack to cool completely.
24. Repeat the process with the next batch of cake batter and cake pans.
25. Once all the cakes are completely cool, you can assemble.

Make the Swiss meringue buttercream:

1. In a stand mixer bowl, add in the egg whites and granulated sugar.
2. Heat the mixture over a double-boiler and whisk with a wire whisk until the mixture heats up to 165°F/74°C.
3. The egg whites will become frothy and hot to the touch (and the sugar should be completely dissolved).
4. Remove the bowl and transfer to the stand mixer.
5. Attach the wire whisk and whip on high speed until full in volume, shiny and glossy.
6. Once the egg white meringue is at room temperature (feel the exterior of the stand mixer bowl), change the whisk to a paddle attachment.
7. On low-medium speed, add in pats of butter one at a time, until incorporated into the mixture.
8. Once all the butter has been added, add in the vanilla extract, increase the speed and whip until the buttercream is light and fluffy.

Assemble and decorate:

1. Place the violet/purple cake layer on a cake round, and onto a cake turntable.
2. Add about ¼ C of buttercream frosting and spread over the surface evenly with an offset spatula.
3. Add the blue layer and repeat, alternating cake layers with frosting.
4. To give the cake some structure since it's quite tall, it's best to add a dowel or long skewer before adding the final (red) layer. (Trim off any excess).
5. Once the red layer has been added, cover the entire cake with a crumb coat, or a thin layer of frosting.
6. Place cake into the fridge to set the frosting.
7. Spread another layer of buttercream frosting and decorate however you like.
8. Cover and store in the fridge until ready to serve.