To make the dough, pour 135ml/4½fl oz/generous ½ cup tepid water into a small bowl and sprinkle over the yeast and sugar. Stir until the sugar dissolves, then leave for 10 minutes until frothy. Sift the flour, salt and baking powder into a large mixing bowl, stir and make a well in the centre. Pour the yeast mixture and 1½ teaspoons of the oil into the well and gradually mix in the flour to form a soft ball of dough. Turn out onto a lightly floured work surface and knead for 10 minutes until you have a smooth and elastic ball of dough. Wipe the inside of the cleaned mixing bowl with a little oil. Add the dough, cover the bowl with cling film and leave in a warm place until doubled in size, about 2 hours.

Meanwhile, make the filling. Heat a wok over a high heat, add the remaining oil and stir-fry the mushrooms and ginger for 8 minutes, or until any liquid evaporates. Spoon the mixture into a bowl and stir in the hoisin, spring onions and cashews. Season with pepper, and leave to one side.

Turn the dough out onto a lightly floured surface. Divide into 8 balls and cover with a damp tea towel. Take a dough ball and press it into a round about 5mm/¼in thick, then put a heaped tablespoonful of the mushroom mixture in the centre of the dough and pull the edges up over the filling, pressing them together to seal. Put on a lightly floured surface and repeat with remaining balls and filling.

Put the buns in a tiered steamer, lined with baking parchment. Cover with a lid and steam for 15 minutes, or until risen and fluffy. (You can cook the buns in batches, if necessary.) Serve the buns warm with extra hoisin sauce for dipping.