When I was small, my family used to take me and my brother for barbeques in a country park in 'Tai Mo Shan' (literally 'Big Hat Mountain'), Hong Kong. Barbeque is a very different affair over there. There are stone barbeques and benches in the park, where people sit around, skewer the food on special metal forks, and grill them over a bed of charcoal. I would always start with two Frankfurter sausages (with a coat of honey) and end with a roast sweet potato. Since purple sweet potatoes are more rare and expensive than the normal ones, my parents would buy a mixture of both and then wrap them in tin foil. And if I was lucky, I would get a purple one.
There are not many Chinese sweet potato desserts. The most common one we eat in Hong Kong is called 番薯糖水 (sweet potato sweet soup) – boiled chunks of sweet potato in ginger syrup. However, cakes made from sweet potato are very popular in Japan, and that is where the influence for this recipe came from. A few weeks ago I got hold of some purple sweet potatoes. I baked them in the oven until soft, and then added... erm... condensed milk. I know it is not very conventional, but it tasted damn good. So trust me on this one.
I used the puree as a filling for a green tea Swiss roll, which I made for last month's Band of Bakers ‘Vegetable’ themed event. The green tea is quite subtle in this recipe, just enough in the background to complement the earthy taste of the sweet potato.
Sweet potato puree
Makes around 300g
3 Purple Sweet Potatoes (around 420g)
4 tbsp Condensed Milk
2 tsp Full Fat Milk
Preheat a fan oven to 200C. Bake the whole purple sweet potatoes (skin on) for at least 50 minutes. Use a fork to poke the potatoes to check that they are soft enough.
One by one, cut the potatoes in half, and then use a teaspoon to scoop out the flesh into a bowl. Use the back of a fork to mash the potatoes. Add the condensed milk and full fat milk. Mix well.
Use cling film to shape the puree into a long tube, twist to tighten both ends and then keep cool in the fridge.
Green tea Swiss roll with purple sweet potato puree
6 Medium Eggs (separated)
100g Caster Sugar
50g Self-Raising Flour
10g Matcha Powder
10g Ground Almond
40ml Full Fat milk
1 tsp Icing Sugar (for dusting)
250ml Double Cream
30g Icing Sugar
160g Purple Sweet Potato Puree
20g Caster Sugar
Preheat oven to 200C. Line a 25cm x 38cm tin with greaseproof paper. Make the sugar syrup by dissolving the sugar in the water, and bring to boil until reduced by half. Leave it aside to cool.
Beat the yolks with 50g caster sugar until pale and creamy. Add half of the flour, mix well. Repeat for the remaining half. Add the ground almond, then slowly sift in the green tea powder, add the milk to loosen the mixture in between.
Using an electric whisk, beat the egg whites at a medium speed until you see lots of small air bubbles. Switch to high speed, and then add the 50g caster sugar, spoon by spoon, into the egg whites. Stop mixing when stiff peaks form. Do not overwork it.
Mix a quarter of the egg white into the cake batter. Fold in the remaining egg whites, a quarter at a time, until fully incorporated. Pour the mixture into the prepared tin, and gently spread out with a spatula. Bake in the preheated oven for 12 to 15 minutes.
Remove the sponge from oven. Cool it in the tin for a few minutes, and then cool (with the greaseproof paper still in place) completely on a wire rack. Place a piece of greaseproof paper (a bit bigger than the size of the tin) on a work surface, turn the sponge upside down.
Carefully remove the paper on the top. This 'smooth' side is the outer layer of the Swiss roll. Turn the sponge over (i.e. 'rough' side up). Brush the sugar syrup onto the sponge, including the edges.
Whip the fresh cream with icing sugar until soft peaks form, and then evenly spread it onto the sponge. With the shorter side facing you, line the puree around 6 to 8 cm from the bottom of the sponge. Roll the sponge over the puree and carry on until it is done.
Use the greaseproof paper to wrap around the roll, twist to tighten both ends and then put in the fridge for at least 30 minutes. Cut both ends off when ready, dust with icing sugar and serve.