Purple Sweet Potato Roll Cake (紅芋ロールケーキ)

I had plenty of purple sweet potato cream leftover as I had made a double batch. I didn't want to use it as a filling of some sort as I had painstickingly took the trouble to strain it. I wanted showcase it, but I didn't want to make another batch of mont blancs. While googling, I found this image and I was inspired to replicate it. I have not much experience making roll cakes, so I decided to try out this recipe that looks pretty good.

Purple Sweet Potato Roll Cake (紅芋ロールケーキ)

The sponge baked up beautifully (if I may say so). Usually, I face a multitude of probems - from underwhipped egg whites (my egg whites tend to reach a foamy stage without going on to the stiff peaks stage), pools of butter at the bottom of the bowl (I didn't fold it in thoroughly), and to streaks of flour bits in the batter (yes, I've learnt that sifting is absolutely necessary).

I've never achieved before baked such a pretty, evenly coloured sponge. My confidence was boosted. And my whipped cream turned out just right too. I was stoked. But, luck doesn't strike thrice I guess, cos the trouble came when it came to piping the sweet potato cream onto the cake. The picture looked easy, so I tried piping in long parallels across the cake. FAIL. The lines didn't fall prettily as it did in the picture, so what resulted was squiggly lines here and there. *tears hair out in frustration* Pardon the ugly piping. I've smothered snow powder on top to try stifle the ugliness so bear with me on the less than perfect piping work. :(

Anyway, I loved the taste and texture of the sponge cake. It was absolutely perfect. Combined with the light cream, and purple sweet potato filling and chunks, it makes a really light and satisfying tea time treat. I can't wait to reuse the recipe again for various flavoured roll cakes! Matcha, yuzu, milk tea, chocolate, coffee. What should I make next?

Purple sweet potato roll cake (紅芋ロールケーキ) (adapted from here)

Sponge
6 egg yolks
5 egg white
100g caster sugar
50g cake flour
50g butter, melted and cooled

Preheat oven to 200C. Line a 30X35 tray with parchment paper.
Put egg yolks and 1 tbsp sugar in a medium bowl. Using an electric mixer, beat at high speed until the yolk mixture is thick, pale and when the whisk is lifted, the batter should fall slowly back into the bowl with a ribbon-like pattern. Set aside.
In another bowl, beat egg whites with an electric mixer until it is slightly foamy. Add the remaining sugar at one go and continue to beat until firm peak forms.
Pour yolk mixture into the meringue. Using a rubber spatula, gently fold the mixture until just combined.
Sift flour over the batter and then fold the mixture lightly but rapidly until the flour has disappeared.
Pour melted butter down side of bowl, and gently fold until completely incorporated.
Pour the batter into the prepared tray, spread the batter evenly with a scraper.
Bake in the preheated oven for 12 minutes.
Lift cake from baking tray onto a wire rack. Let it cool slightly, cover the cake with a piece of cling wrap to cool completely.

Filling
300ml whipping cream
3 tbsp icing sugar
1 tsp vanilla paste

Add cream and sugar in a medium bowl, put it into a bigger bowl filled with iced water. Whisk the mixture until firm peak forms.

Assemble
1 purple sweet potato, diced
leftover purple sweet potato cream (recipe here)

Remove liner from the cake. Transfer the cake onto a piece of parchment paper with the baked side facing upwards.
Spread the filling evenly on the sponge base (leaving about 2/3 - 3/4 cup of cream filling aside). Place the diced sweet potato on top of the filling. Roll the cake up and with then tuck in both ends of the parchment paper. Put the roll into the fridge to chill and set. After half hour, spread the remaining cream filling onto the outside of the roll. Return to fridge to chill for a few hours.
Place purple sweet potato in piping bag with Wilton multi-opening decorating tip attached and pipe purple sweet potato cream onto the top of the cake. Sift snow powder right before serving and top with purple sweet potato slices (optional). Slice and serve.