In a large bowl, mix flour, caster sugar, and ½ tsp salt.
Dissolve yeast and a pinch of sugar in 1 tbsp warm water.
Add this yeast mixture, along with milk, sunflower oil, rice vinegar, and 200ml water, to the flour mixture.
Stir until a dough forms, adding more water if necessary.
Turn the dough onto a lightly floured surface and knead for 10-15 minutes until smooth.
Place in a lightly oiled bowl, cover with a damp cloth, and let rise for 2 hours, or until doubled in size.
Punch down the dough on a clean surface, flatten it, and sprinkle with baking powder.
Knead for an additional 5 minutes.
Roll the dough into a 3cm thick sausage shape, then cut into 18 pieces.
Roll each piece into a ball and let rest for 2-3 minutes.
Roll each ball into an oval about 3-4mm thick.
Rub each oval with oil, place an oiled chopstick in the center, fold the dough over it, and then remove the chopstick.
Place each bun on a square of baking parchment on a baking tray.
Cover with a tea towel and let rise for 1 hour 30 minutes, or until doubled in size.
Steam the buns in a large steamer over medium-high heat for 8 minutes, until puffed and cooked through.