Mix together 525g plain flour, 1½ tbsp caster sugar and ½ tsp salt in a large bowl. Dissolve 1 tsp fast-action dried yeast and a pinch of sugar in 1 tbsp warm water, then add it to the flour with 50ml milk, 1 tbsp sunflower oil, 1 tbsp rice vinegar and 200ml water. Mix into a dough, adding a little extra water if needed. Tip the dough onto a lightly floured work surface and knead for 10-15 mins, or until smooth. Place in a lightly oiled bowl, cover with a damp cloth and leave to rise for 2 hrs, or until doubled in size.
Tip the dough out onto a clean work surface and punch it down. Flatten the dough with your hands, then sprinkle over 1 tsp baking powder and knead for 5 mins. Roll out the dough into a long sausage shape, about 3cm thick, then cut into pieces that are about 3cm wide – you should have 18. In the palm of your hand, roll each piece of dough into a ball and leave to rest for 2-3 mins. Use a rolling pin to roll out each ball, one by one, into an oval shape about 3-4mm thick. Rub the surface of the dough ovals with oil and brush a little oil over a chopstick.
Place the oiled chopstick in the centre of each oval. Fold the dough over the chopstick, then slowly pull out the chopstick. Cut 18 squares of baking parchment and put a bun on each. Transfer to a baking tray, cover with a clean tea towel or lightly oiled cling film and leave to prove in a warm place for 1 hr 30 mins, or until doubled in size. Heat a large steamer over a medium-high heat. Steam the buns for 8 mins until puffed up (you’ll need to do this in batches).
Meanwhile make the filling. In a wok, add some oil and sizzle for 30 seconds before adding the beef strips. Add the hoisin sauce and mix to combine. Once cooked, prise open each bun and fill with our hoisin beef and cucumber matchsticks, and serve with some salad. Eat while they’re still warm.