I've been making this sponge cake for years. This is my special recipe that's so delicious, I almost want to keep it a secret. The decorated cakes I make with this sponge cake are nearly cake shop quality!
[Preparation] Line a cake mold with parchment paper. (Not greased with butter)
Combine the eggs, egg yolk and granulated sugar in a bowl. Combine the butter and vegetable oil in a small bowl, and combine the cake flour and corn starch in a different bowl.
Mix the eggs and granulated sugar lightly with a whisk and heat to body temperature in a double boiler.
Change to a hand mixer, and whip at the highest speed until the mixture becomes whitish and forms a thick ribbon when the hand mixer is lifted. Some of the batter should remain within the whisk when lifted.
Once the mixture has reached the consistency of Step 4, stir slowly at low speed for about 4 minutes. This is just to break big air bubbles, so don't stir too much.
Sift in the dry ingredients into Step 5 and fold in gently with a rubber spatula until the mixture is no longer floury. (About 30 times.)
Heat up the butter and vegetable oil to 50 ℃ while mixing the flour.
Add a scoop of the mixed batter from the Step 6 into the oil mixture from Step 7 and mix well with the whisk.
Meanwhile, preheat the oven to 170℃.
Pour the mixture from Step 8 into the rest of the batter from Step 6. Change to a rubber spatula and mix from the bottom until the batter becomes shiny.
When mixed, pour the batter into the mold, and tap it on the counter once to remove air.
Bake for about 25 minutes at 170℃. (For a 18 cm mold, bake for 30~35 minutes.) This photo shows how the cake looks while baking. It smells really nice.
When it's baked, drop it from a height of 30 cm once, and place upside down on a cake rack that is lined with parchment paper and let it cool.
When it's completely cooled, it's done. If you don't use it straight a way, wrap with cling film over the parchment paper and transfer to the fridge.
This is how it looks when cut. You can bake a fine textured sponge cake that is moist and fluffy.
Story Behind this Recipe
I experimented with various ratios of the dry ingredients and finally came up with this sponge cake.
If you raise the temperature of the egg mixture too high before you whip it, you will end up with a lot of air bubbles and the batter will be lumpy. So cool it down if it's too hot. If you have a thermometer, check the temperature of the butter before you add it to the batter. This process has an impact on the shape of the cake when it's baked (the dome on top falls depending on the temperature of the batter).