Purple Sweet Potato Roll Cake with Vanilla Greek Yoghurt Filling

Seems like I am not quite done with my swiss roll craze! I am still rolling away and am certainly enjoying each roll better :). Getting the hang of baking and rolling this cake which was on once a big challenge for me.

Cracked cake, sticky tops, I had them all but I am glad I made it through with practice (and no giving ups!). I got to know what is the comfortable temperature and time to turn out a good cake, easy for me to handle and of cuz yummy enough for the tummies.

So, what's with the purple sweet potato in my roll cake? It was not in my plan to bake this actually but as I was peeling some purple sweet potatoes and putting it to steam for my breakfast, I thought I could try making some purple roll cake with it! Sadly the cake did not turn out to be a nice purple hue that I imagined it to be. It was still good anyway.

I was lazy to beat up butter or whipped cream for the cake filling, so I used greek yoghurt instead. With a slight sour twist to it, it gave the cake a refreshing feel since the purple sweet potato (seriously) did not contribute much taste to the cake.  I enjoyed it anyway :)

Purple Sweet Potato Roll Cake with Vanilla Greek Yoghurt Filling

60g egg yolk
30g canola oil
15g caster sugar
45g milk
50g mashed purple sweet potato (steam sweet potato and mash it right after it is dteamed and still hot)
65g plain flour

160g egg white
70g caster sugar

1. Whisk egg yolk and sugar till well mixed and sugar is dissolved.

2. Add in canola oil and milk and whisk to combine.
3. Add in mashed purple sweet potato and whisk well to break up potato mash and incorporate it well with egg yolk mixture.
4. Sieve plain flour into mixture and whisk to stir till combined.
5. With an electric mixer, beat egg whites till foamy. Add in caster sugar in 3 additions, making sure each addition is well mixed before adding more. Beat till stiff peaks form.

6. Place 1/3 of egg white into yolk mixture and fold in gently with a spatula or a whisk.
7. Fold in remaining egg whites in 2 additions till just incorporated. Make sure not to over fold and deflate the chiffon mixture.
8. Pour chiffon mixture into lined square tin (10"×10"), then use a scraper to spread out and level mixture.
9. Put to bake in preheated oven at 180deg C for 12 to 15min or till done. (I baked 180deg, 21min in my Rowenta oven) To test done-ness, when cake springs back and makes a "sss" sound when pressed, it is done.
10. Remove and place on cooling rack. Place a new baking paper over top of cake and flip over to other side of cake. Remove baking paper and let it cool slightly before  rolling. You can choose to roll cake on either side of the cake.
11. Slice off end of swiss roll to form somehwat 45deg angle so that when cake is rolled till the end, it gets tucked nicely underneath. At the front of the cake where you will start the rolling, make 2 horizontal cut gently on the cake to allow easier rolling. Take care not to slice through cake.

12. Spread your favourite fillings and start rolling! (For me, I used vanilla greek yoghurt.)
13. Wrap rolled cake in baking paper and leave in fridge to set its shape for about 45min before slicing.