Black sugar buns (manjū)

Instructions

1. The day before you plan on making the buns, place adzuki in a large bowl or container, cover with water by 2 cm and leave to soak overnight.
2. Drain the beans in a colander and rinse with running water, then transfer to a large saucepan along with 1 litre water. Bring to a boil over medium heat, then reduce the heat to low and simmer for 20 minutes. Drain.
3. In a clean saucepan over low heat, bring 1 litre water to a simmer, then add the sugar. Add the drained beans to the saucepan and simmer for 45-60 minutes, or until the beans come apart easily when squeezed. Remove from the heat, allow to cool and store in an airtight container in the fridge until needed.
4. Place the red-bean paste in a large saucepan and bring to the boil over medium heat. Simmer until the mixture is thick enough that a line drawn through the mixture using a wooden spoon remains. Remove from the heat and allow to cool.
5. Scoop out 20 evenly sized portions of the cooled red-bean paste and roll them into balls, then set aside.
6. In a small, clean saucepan, combine the black sugar, white sugar and 50 ml (1¾ fl oz) water. Bring to a simmer over low heat and cook until the sugars are dissolved. Remove from the heat and cool to room temperature, then add the flour, potato starch and bicarbonate of soda and knead into a smooth and pliable dough.
7. Preheat the oven to 180°C (350°F) and line a baking tray with baking paper.
8. Divide the dough into 20 pieces. Flatten each piece into a disc, then place a ball of red-bean paste in the centre of each disc. Wrap the dough around the red-bean paste, pinching it together to seal, and shape it into a smooth oblong bun. Place each bun on the baking tray, seam side down.
9. In a small bowl, beat the egg and milk together to make a wash and brush it over the buns.
10. Place the buns in the oven and reduce the heat to 130°C (270°F), then bake for 15 minutes. Cool on a wire rack and serve.