In a large bowl, mix the flour, sugar, yeast and 1/ 4 tsp salt with 180ml lukewarm water. Knead the mixture for 10 mins or until elastic. Put in a lightly oiled bowl, cover with a damp tea towel and leave in a warm place to prove until it has doubled in size, about 1 1/ 2 hrs.
Meanwhile, in a small bowl, mix the seasoning ingredients and 1/ 4 tsp salt. In a separate small bowl, mix the cornflour with 2 tbsp cold water and set aside.
In a wok over a medium heat, fry the ginger with 1 tbsp rapeseed oil for 30 secs. Add the mushrooms and the seasoning mixture and stir-fry for 1 min. Pour the cornflour paste into the wok and stir-fry for 1 min more, or until the sauce thickens.
Add the Chinese cabbage and fry briefly, then remove the wok from the heat. Add the noodles, tofu, spring onions, sweetcorn and sesame oil. Gently fold everything together so that the filling is evenly mixed, then leave to cool.
Punch the air out of the dough and divide into 20 small balls about the size of a golf ball, each weighing roughly 25g. Put them in a roasting tin and re-cover with the tea towel. Leave in a warm place to prove for a further 20 mins.
Flatten the dough balls with your hands. Use the tip of a rolling pin to roll around the edge of the dough ball a few times to flatten it, then gently work around the edges using your fingertips. The aim is to achieve a round sheet of dough with a thicker bump in the middle on which you can place the filling. Repeat until all of the balls have been rolled. Cover them again with the damp tea towel as you go so they don’t dry out.
Taking a dough sheet in the palm of your hand, put 2 tsp of the filling in the middle. Cup your palm so the filling remains in the middle, then gently gather up the edges into the middle and twist to seal. Continue until all the buns are filled. Put them back in the roasting tin and re-cover with the tea towel again
Add 1 tbsp rapeseed oil to a frying pan over a medium heat. Arrange half the buns in the pan, leaving 1/ 2 cm between them so they have enough room to rise during cooking. Fry for 1-2 mins, sprinkling with a little water, then pour 100ml water into the pan and cover with a tight-fitting lid.
Cook for 2-3 mins until all the water has evaporated and the buns have doubled in size. They should become very white and the bottom should be crispy and golden brown. Repeat the process with the remaining 1 tbsp oil and the rest of the buns. Mix the sriracha and oyster sauce together in a small dish and serve with the buns.