• You need at least 2 round or square cake pans of the same size for easier work flow. If you have three, that's even better. It helps to speed things up.
• Preheat oven to 350 F (180 C). My oven is conventional with bottom heat only. If you have a convection oven, I suggest lowering the temperature by 25 F (15 C). Line the bottom of the pan with parchment paper

If you have three pans (which is my scenario):

• I have three square pans of the same size and that's what I used. If you use three pans, you can bake the cake 3 trays at a time and only two rounds of baking and you will whip the egg whites twice separately because the meringue can't sit on the counter waiting for the cake to bake
• Separate the eggs when they are still cold, they are easier to separate. Put two egg whites in one bowl and another two in a different bowl. Make sure the bowls are grease-free. Let them come to a room temperature

If you only have two cake pans:

• You will bake the cake 2 trays at a time and three rounds of baking and you will whip the egg whites three times separately because the meringue can't sit on the counter waiting for the cake to bake
• Separate the eggs when they are still cold, they are easier to separate. Measure the total weight of the egg whites and then, whisk it a bit so you they are easier to divide. Divide into 3 different bowls. Make sure the bowls are grease-free. Let them come to a room temperature

Prepare the yolk batter:

• Mix cake flour and salt and whisk to combine. In a large mixing bowl, combine egg yolks, sugar, oil, milk, and vanilla extract. Whisk to combine. Sift in cake flour and salt. Use a zig zag motion to combine everything. The yolk batter should be thick but flowy
• Divide the batter into 6 different medium to large size bowl. When I weighed, it's about 56-60 grams each portion. Depending on how intense or soft the color you want, add the food gel coloring on each batter. I did purple, blue, orange, yellow, green, and pink/red

Prepare the meringue:

• Pour the first portion of egg whites in a mixing bowl of a stand mixer fitted with a whisk attachment or you can use a hand mixer too. Whip the egg whites until foamy, about 1 minute over medium speed (speed 4) and then add the acid and whip for another minute. Add 1/3 of the sugar and beat on higher speed (speed 6) for one minute then add another 1/3 until you run out of sugar. Whip the meringue until stiff peak. Stop halfway to check the consistency so you don't overwhip the meringue. When you lift up the batter with the whisk, it should stand up firmly with just a tiny bent on the tip. Stop beating. If you beat further, it will break and turn foamy

Combine meringue and cake batter:

• Get about 1/3 of the meringue (about 35 grams) and put into one of the yolk batter and start folding the meringue with the batter using a swipe down and fold over motion until the two are combined and increase in volume. You shouldn't see any more white meringue. Pour this into one of the prepared cake pan. Tap on the counter a few times to pop large bubbles
• Repeat with the next 1/3 of the meringue and put into another yolk batter and continue folding to combine meringue and the yolk batter and pour into the another prepared cake pan. Do the same with the last 1/3

Bake the first three cakes:

• Now you have three cake pans filled with cake batter. Put them in the oven, in the middle rack, side by side and bake for 15 minutes or until a cake tester inserted comes out clean and the top is no longer sticky when you touch it
• When they are done baking, remove from the oven and drop the cake pan on the counter a few times to prevent shrinkage. Let them cool down in the pan for 5 minutes and then remove from the cake pan and place the cake on cooling rack to cool down completely
• Clean these three pans and line with parchment paper again to get ready to bake the next three cakes

Repeat the process again for another three cakes:

• Pour the second portion of egg whites in a mixing bowl of a stand mixer fitted with a whisk attachment or you can use a hand mixer and repeat the process again. If you only have two pans, then you will repeat this process one more time later
• Combine the meringue with the yolk batter again following the steps I mentioned above. Pour into the cake pans and bake these last three side by side at 350 F (180 C) for 15 minutes

Cooling down:

• Let all 6 cakes cooled down completely before you decorate them. You can wrap them up with a plastic wrap after they have cooled down completely and keep at room temperature until the next day to decorate them
• You can also freeze them for up to one month and simply thaw at room temperature and decorate them. I've done that before and they turned out well. Freezing them is better than keeping them in the fridge, which will dry out the cake

If you choose stabilized whipped cream using cornstarch:

• Chill your mixing bowl and whisk attachment in the fridge for 15 minutes. The cream whip better when everything is cold. Make sure the heavy cream has been chilled in the fridge too
• Pour the heavy cream, icing sugar, and vanilla extract into a mixing bowl. Start whipping at medium speed until it reaches a firm peak, meaning it holds its shape, but don't overwhip until the mixture turn grainy
• This can be store in an air-tight container for about 2-3 days. The whipped cream may not last as long at room temperature compared to the one stabilized with gelatin and cream cheese frosting, but it should be good for about 2-3 hours at room temperature

If you choose stabilized whipped cream using gelatin (must be used immediately)

• Please take note: The whipped cream frosting need to be used immediately before the gelatin sets
• Mix the gelatin with water to let it soften for 5 minutes. Melt in the microwave for 5 minutes. If it's still not melted, microwave for another 3-4 seconds. The gelatin will be clear when it melts. You can also put the bowl with the gelatin in a warm water to let the gelatin melts
• Chill your mixing bowl and whisk attachment in the fridge for 15 minutes. The cream whip better when everything is cold. Make sure the heavy cream has been chilled in the fridge too
• Pour the heavy cream, icing sugar, and vanilla extract into a mixing bowl. Start whipping at medium speed until it reaches a very soft peak that barely holds its shape at all. Drizzle in the gelatin and continue whipping over medium speed until it is firm and can hold its shape but not overly firm that it turns grainy and chunky. You have gone too far. This has to be used immediately before the gelatin sets

Assembling the cake:

• Make a simple syrup by mixing sugar with hot water to melt the sugar. The simple syrup will keep the cake moist. This is optional if you don't want to do it. I did it
• The order of the cake should start with purple, blue, green, orange, yellow, pink/red. Dab a small amount of frosting in the middle of the cake stand, cake base, serving plate of whatever you use. So place the purple cake (bottom side facing up). Brush with simple syrup. Apply a thin layer of frosting. Try to spread it out evenly to the edge with offset spatula. Place the blue layer and then repeat all the way until you get to the top layer, red.
• Now get ready to frost the side. Apply a thin layer of frosting all around the sides, we call this crumb coat. This is to seal off any loose crumbs. You will still see the cake, it's okay, we're not done yet. Place the whole cake inside the fridge for about 30 minutes. This will help to set the crumb coat. Remove from the refrigerator and then generously coat the rest of the cake, starting with the sides and then the top. Use your offset spatula or scraper to smooth out the frosting as evenly as you can. If you want to add any decoration to the cake, this is the time to do it. Place the cake back into the refrigerator for at least 30 minutes, to let the frosting set before you cut it. It will have a nice more defined layers