Ladies and gentlemen, please meet my seized chocolate cake:
Don't worry, I haven't seized anything from anyone. It's cocoa powder that's doing the seizing, not me.
My very soft and fluffy chocolate cake is made with cocoa powder that's mixed with hot oil. The scalding helps bring out the chocolate flavour. Of course, I use high quality cocoa powder or there wouldn't be any flavour to bring out.
When the oil and cocoa combo is not too hot and not too cold, I add a little bit of milk. This is when the seizing happens, i.e. the small cocoa particles absorb the milk, become sticky, and stick to one another to form bigger particles, resulting in a thick paste. The thickness of the cocoa paste is crucial to the success of chocolate sponge cake.
If the cocoa powder isn't seized at all, it'd be suspended in a runny liquid, which will sink to the bottom of the pan during baking. The cake will be pale and bland except for the bottommost 2 mm or so. If that thin layer is stuck to the pan or parchment paper after unmoulding, then the cake doesn't taste of chocolate at all.
If the cocoa powder is "over-seized", it becomes too coarse and will look like ground black pepper in the cake. The colour of the crumb, light brown with specks of black, won't look right. There will be some chocolate flavour but it'll be weak.
When the thickness of the cocoa paste is just right, the chocolate flavour is strong and spread evenly in the cake. And the cake's colour is a nice medium-brown, not pale.
How do you control how much the cocoa powder seizes? By watching the temperature. The hotter the oil and cocoa powder mixture is when milk is added, the stronger the seizing. The right moment for adding the milk is when the bowl holding the mixture doesn't feel hot but is still quite warm.
Of the various types of cake I bake, chocolate sponge rises the most, more than even chiffon cakes. The batter more than doubles in height in the oven. After shrinking a bit whilst cooling down, the cake is twice as tall as before baking. The more the batter rises, the fluffier the cake, right?
To make the batter rise as much as possible, the bottom of the pan should have more heat than the top. Separate controls for the oven's top and bottom heating elements would be very handy. If your oven isn't so fancy, as mine isn't, just bake the cake on the bottom instead of middle shelf.
After baking comes eating. When the chocolate sponge is unadorned and still warm, the fluffiness really shows through. If you are a chocolate fiend, you could cover or sandwich the cake with ganache. Whipped cream or buttercream, whether plain or chocolate-flavoured, is quite nice too. Do you like black forest cake? A sexed up chocolate cake would be ideal for a celebration.
Compared to my vanilla sponge and pandan sponge recipes, chocolate sponge is easier. The batter is more stable because it has less egg white, so it doesn't deflate easily. With this recipe, I think even a novice baker can bake a chocolate cake that's soft, fluffy, moist and chocolatey . . . if he/she follows the instructions. That's not too difficult . . . is it?
12 December 2013 Update
Here's a video to explain why glucose helps make cakes fluffy: