This recipe comes from the same cookbook as the 5-coloured glutinous rice balls in milk soup I made earlier, a book of Japanese autumn & winter wagashi entitled 《和果子．和甘味 (秋冬篇)》 (in Chinese and English).
200g yellow core sweet potato [used orange flesh type instead]
200g purple core sweet potato [found this Thai-origin variety at Sheng Siong]
4 Tbs butter [used coconut oil for added flavour & aroma, but effect was rather strong]
2 Tbs milk [used more than this]
4 Tbs granulated sugar [omitted as sweet potatoes already sweet]
1 Tbs rum [omitted]
1) Peel, rinse and slice sweet potatoes and steam until well cooked, about 15 mins
2) Separate the two kinds of sweet potatoes and mash them in two separate bowls.
3) Divide other ingredients into two portions and add to each bowl of mashed sweet potato. Some varieties of sweet potatoes (such as Japanese sweet potatoes, satsumaimo) have lower water content and more milk is required to get a smooth paste. For the purple sweet potato, I had to use quite a bit more than 1 Tbs. Added a bit at a time until the texture of the puree looked OK.
4) Sieve each colour of sweet potato puree by forcing through a large sieve with a wooden spoon so as to remove coarse fibres. I found the orange sweet potato to be especially fibrous. Be careful not to contaminate the sweet potato with the other colour.
5) Take a wet cloth and put 10g of each kind of sweet potato in the centre. Wrap up and twist the top then shape the sweet potato into the shape of a ball. Repeat until all the sweet potato is used up.
Verdict: I enjoyed the subtle natural flavours of the sweet potato, but the 2Tbs of coconut oil was rather overwhelming, perhaps I’ll use half butter next time.
The sieving took some patience but the hardest part was carefully forming the dual-coloured balls into a presentable looking shape. The balls are very soft so once you have removed them from the cloth, you can’t touch them again or their beautiful shape will be destroyed. I made up just enough balls for immediate consumption then stored the leftover sweet potato puree in two separate containers in the fridge.