Purple Sweet Potato Roll Cake – Recipe

"Purple potatoes?" Asked Dad, incredulous.

"Yes," I replied patiently, "If it helps, just pretend it's orange like normal sweet potatoes- the only difference is the colour."

"Hang on hang on, I'm having a hard enough time believing you could even make a cake out of potatoes!" Laughed Dad.

"Of course you can," I said, "What about all those cakes that are safe to eat for Passover? What do you think they're made out of? Potato flour!"

"I don't know about that..." Said Dad, staring at the offending purple cake somewhat suspiciously.

I've been wanting to work with purple sweet potatoes since I proceeded to burn one in the process of making bubur cha cha- unfortunately when I went back to the store I found them in Chinatown soon after, they were no longer selling them. Thankfully they're back in stock again- only it's way too summery and hot outside for a hot dessert like bubur cha cha, now. So I made this instead- finished with strawberries and a whipped cream cheese filling.

In the end, this cake was very well-received: once you get over the shock of colour and that it's actually all natural, you can appreciate the mild, almost nutty sweet flavour of the sweet potatoes. Both Mum and Dad loved it, and reckoned that if I hadn't said anything, they'd have never guessed it was made from potatoes.

Best of all, look at the FANTASTIC colour of these things:

And it's all natural. Awesome.

If you can't find these beauties (I can only ever find them in one particular shop in Chinatown), you can substitute them for regular orange sweet potatoes.

Ingredients for Swiss Roll:

-250g sweet potatoes in any colour you can get your hands on
-4 eggs, separated
-40g caster sugar
-30g self-raising flour, sifted (you don't need much because most of the starch comes from the potatoes)
-2tbsp milk
-1tbsp vegetable oil

Ingredients for Filling and Topping:

-300ml whipping cream
-200g cream cheese
-5tbsp caster sugar
-300g strawberries

Method:

1) Start by peeling and chopping up your sweet potato(es) into small cubes and steam them for 15-20mins until tender (steaming rather than boiling keeps the best colour).

2) Mash potatoes until very smooth and let them cool (if you have a coarse sieve, press them through that after roughly mashing).

IT'S SO PURPLE

3) Preheat the oven to 180 degrees C/ Gas Mark 4.

4) Line a 26x36cm baking tray with non-stick greaseproof paper and lightly grease it.

5)  Whisk the egg whites in a large bowl until they form a stiff peaks when you lift the beater out, add half of the sugar and continue to beat until glossy and white (this can take a few minutes).

6) Whisk the other half of the sugar with the egg yolks in a separate bowl until pale and fluffy (be sure to do the yolks AFTER the whites, unless you clean the beaters in between).

7) Add in the oil, milk and potato purée and whisk until smooth.

8) Gently mix in the flour until just combined.

9) Fold a couple of tablespoonfuls of the egg white meringue mixture into the potato mixture to loosen it up a bit, then very gently but thoroughly fold in the rest of the egg whites, again until just combined.

10) Pour into your prepared baking tin and spread evenly, dropping the whole tin down on your counter a few times to bash out large air bubbles, and bake for 15-20 minutes until a skewer comes out clean when you poke the cake with it.

11) Turn out onto a baking parchment-lined cooling rack, leaving the tin on top whilst it cools to keep in the moisture.

12) Whisk the cream cheese with the sugar until fluffy, and add the whipping cream, whisking until very thick (careful not to overdo it or you'll make cheesy butter). Quarter the strawberries, leaving six whole to garnish.

13) When completely cool, remove the tin from the cake and peel off the top layer of baking parchment. Lay the parchment back over the top and flip the whole thing over (so that the parchment it was baked with is at the bottom, and the parchment you used to line the rack is at the top). Get rid of the parchment that's now on top.

14) Cut a bit off the furthest end of the cake, sculpting at a sloping angle so that it'll roll up more snugly.

15) Spread the your whipped cream cheese on, leaving a bit of space around the sides and towards the end (because the cream will squish out as you roll the cake up). Arrange your strawberries on top of the cream, and very tightly roll up the cake, using the parchment underneath to help you. NOTE: You'll have some of the cream left to garnish, so put it in a small bowl and in the fridge until you need it.

16) Once rolled up, twist the ends of the parchment like a sweet wrapper to keep the cake firmly rolled up. Cover it with clingfilm and chill in the fridge for at least four hours.

17) When you're ready to serve, take the cake out of the fridge, remove all the wrapping and place it on a plate. Pipe on some of the remaining cream mixture, garnish with your whole strawberries, and slice!

Here, I have tried to pipe pretty swirls of cream. Alas, as it reached 28 degrees C in the kitchen whilst I was doing this, the cream kept melting as I tried to pipe it. So dollops it was.

This cake is wonderfully melt-in-the-mouth: even if you can't find purple sweet potatoes, do try it with the regular kind.