Every now and then you come across an ingredient that you fall head-over-heels in love with, you cook everything imaginable with it and spend hours dreaming about how to get just one more recipe out of it. For us, that ingredient is murasaki imo, or purple sweet potato. Similar to the pale yellow or white fleshed sweet potatoes usually favoured by the Japanese, these purple potatoes have more dry matter than the orange fleshed American variety, and a much stronger taste. A rich, fruity, almost winey flavour, an otherworldly, deep purple colour and the added bonus that they are packed full of vitamins make them a winner in our books for savoury dishes or desserts.
Japanese meals do not traditionally have a dessert course or end with something sweet. The time for a sweet treat is at around either 10am or 3pm, as a contrasting flavour to go with the slightly bitter green tea that workers would normally stop for. The confections served with tea vary from moulded higashi of sugar and rice flour to fresh fruits, and from jelly-like warabi mochi made from bracken starch to small French style cakes and tarts such as this Okinawan creation. Murasaki imo is mashed, enriched with cream, butter and sugar then piped into crisp pastry cases, just enough for three or four bites before you get back to work.
Pastry case ingredients.
- 60g butter at room temperature
- 45g caster sugar
- 125g plain flour
- 1/2 a beaten egg
- pinch of fine sea salt
- 1 vanilla pod
- In a large bowl, cream together the soft butter and sugar with a spatula. Using the point of a sharp knife, slit down the side of the vanilla pod, open it up like a book and scrape out the seeds, add these and the beaten egg to the butter and mix until thoroughly incorporated.
- Sift the flour and salt together. Add a third of the flour to the batter and stir in well, repeat this for the second third of the flour. Spread the rest of the flour onto a work surface and tip the pastry mixture onto the floured area. Using your hands now, bring the pastry together, without working it too much, until you have a cohesive dough, adding a little more flour if you feel you need it.
- Wrap the pastry in cling film and refrigerate for at least an hour before using it.
- On a floured surface, roll out the pastry to around 3mm thick. Grease twelve 12cm by 6cm oval tins with butter, then line these with the pastry. Cut the top of the pastry even with the tins and prick the base of the pastry with a fork to prevent it rising too much.
- Bake the pastry cases for 10-12 minutes in a 190ºC oven until a light golden brown colour all over. Leave the cases to cool for five minutes before removing them from the tins and placing on a wire rack to cool completely.
Murasaki Imo filling ingredients.
- 600g murasaki imo
- 30g butter
- 75g sugar
- 60ml double cream
- 1/4 teaspoon fine sea salt
- Scrub the sweet potatoes and remove any roots or eyes from them, leaving the skin on. Next, steam them whole for approximately 30 minutes until a skewer can be pushed through with minimal resistance. Drain the water away and allow the potatoes to cool enough for you to handle.
- Carefully peel the potatoes, discarding any fibrous bits you come across, then mash the flesh into a smooth, bright purple paste. Pass the mashed potato through a sieve to ensure it is completely free of lumps, then return it to the saucepan.
- Add the remaining ingredients to the potato, place the pan over a low heat, and slowly warm to melt the butter. Cook the murasaki imo for fifteen minutes, stirring constantly with a spatula, until it has thickened substantially and taken on a deep glossy sheen. This will intensify the flavour of the sweet potato and give it a more toothsome texture. Cover the mixture with some greaseproof paper to prevent a skin forming and allow it to cool completely.
- Prepare a piping bag with a star shaped tip. Gently beat the filling mixture with a spatula to make sure it’s smooth, then fill the piping bag. Using a slow, even pressure, fill each of the pastry cases with the potato mixture. When all the cases are full, let them rest for ten minutes before serving.
Makes 12 tartlets.