Purple sweet potato or sometime known as Okinawan sweet potato is a thing of beauty. It has a similar flavour and sweetness as the ubiquitous orange variety but once cut into it, revealed an intoxicatingly vivid purple hues.
Do not confuse it with Ube/Ubi (purple yam). Although both has similarly coloured flesh and both are frequently found in many puddings and sweets across Asia. Ube is more commonly used in the Philippines where this purple tuber are transformed into many of their nation favourite treats. It has a slight darker and rougher looking skin and is harvested above ground. Okinawan sweet potato on the other hand, is grown underground.
I'm a huge fan of this purple variety as growing up, there would often be many wonderful treats made using this, like Bubor Cha Cha and all sorts of Nyonya Kueh(cakes) such as these Kueh Talam.
Traditionally, these sticky cakes would be made using pandan leaves and coconut milk, yielding its signature emerald green and white layers. My nan however, being a purple sweet potato fan herself, would often give these kueh a beautiful purple twist which is just as attractive to look at and more importantly, equally delightful to eat. The sweetness of the potatoes and the fragrant coconut milk makes this chewy little cakes all the more enjoyable.
The purple sweet potatoes are not that common but can be easily found in most good Chinese supermarket. I usually get mine from Chinatown.
I have faithfully stuck to the dainty version that my nan would make, using tiny Chinese teacups which has been painstakingly oiled before each steaming process. However, you can use a cake tin for this and cut into small bites with a well oiled knife instead. Just make sure you increase the steaming time appropriately and to test, stick a chopstick or skewer into the middle of the cake and it should come out clean when done.
Ingredients (Makes about 28-30 small cakes or fills a 8-in square cake tin)
For the bottom layer:
300g purple sweet potatoes
350ml coconut milk
150g tapioca flour, sifted
1/2 tsp salt
150g caster sugar
For the top layer:
450ml coconut milk
100g tapioca flour
2 tbsp rice flour
1 tsp salt
sunflower oil, for greasing
Peeled the sweet potatoes and cut into thick slices. Place in a prepared steamer and steam for 15-20 minutes, until cooked through.
Put the cooked sweet potatoes into a large bowl and mashed till smooth before adding the coconut milk and mix well. Add the tapioca flour, salt and sugar and stir to combine.
Pour the batter into a saucepan and cook over low heat, stirring constantly, until just warm through so that the sugar will dissolve.
Lightly greased the small cups (or a cake time if you are using) with the oil before filling them with the batter until just slightly over the halfway mark.
Prepare the steamer and steam for 3 minutes until the batter is firm and turns a darker, translucent shade. (Increase the cooking time if you are making this in a cake tin to 20-25 minutes)
While the bottom layer is in the steamer, prepare the top layer batter. Combine the coconut milk, tapioca flour, rice flour and salt in a large bowl and mix well.
Spoon this over the steamed purple layer and steam for another 3-4 minutes until the white layer is cooked through and set. (Again increase this to 20-25 minutes if using a cake tin)
Allow the kueh/cake to cool completely before turning them out. If using cake tin, cut into bite sizes to serve.