Bolu Kukus (Indonesian Steamed Cake) - Revisited

I've made this before. "bolu" is the more correct way to call this gems, because it means "cake". "kukus" is "steamed", so, yeah, this is a steamed cake. I modified the recipe a bit because I plan to use no more than 100 gr of flour. If I literally halved the recipe, I'd have to use 1 1/2 egg yolks and 1/2 white. My goodness, I don't care about halving an egg yolk!

The only problem I got was the Sprite. I couldn't buy only a single can of Sprite in Woolworths, so I used Solo instead. Somehow the batter was thicker, bubblier, but less sweet than the one I made before (using Sprite in Indonesia). I guess that is due to the carbonate content - I once heard that soft drinks in Indonesia have the lowest carbonate content compared to the other countries.

Bolu kukus
yields 5
1 egg
90 gr caster sugar
90 gr plain flour
50 ml lemon-flavoured soft drink (I'll try 60 ml when using Solo next time)
1-2 tsp cocoa powder

1. Prepare steamer. Line roti kukus moulds with parchment paper.
2. Beat egg and caster sugar until thick and pale.
3. Fold in 1/3 of the flour, then 1/2 of the sprite. Gently mix. Repeat with remaining flour and sprite, ending with flour. Mix thoroughly until batter is free from lumps.
4. Divide 1/3 of the batter into a small bowl and add cocoa powder. Whisk until thoroughly combined.
5. Spoon white batter into moulds. Swirl a little of the chocolate batter. Steam for 20 minutes on high heat.

Notes:
- it is okay to steam the cakes in several batches - it is best to steam them in small quantity at a time. Unlike common cake batters which should be baked immediately, this batter can stand quite long.
- do not open the steamer at the first fifteen minutes. Peeking can cause the rising of the cake to stop.
- some people use the usual muffin pans for this cake and it turns nice too... haven't tried it myself though.