Begin the wrappers. Sift the flour, cornflour and salt into a large bowl. Make a well in the centre and add the sesame oil, vegetable oil and 5 tablespoons of water. Use your fingers to mix the ingredients to a stiff dough, adding a little more water if necessary. Turn out the dough onto the work surface and knead it until smooth, then wrap it and leave it to rest for 30 minutes.
Make the filling. While the dough is resting, place the dried mushrooms and the noodles in a bowl and cover them with the boiling water. Leave them to soak for 15 minutes, until soft, then drain them and squeeze out any excess liquid from the mushrooms. Slice the mushrooms into thin slices and the noodles into about 5cm lengths.
Heat a wok or large frying pan over a high heat until it is hot. Add the chilli oil and, when it’s very hot, add the garlic, ginger and spring onions and stir-fry for 10 seconds. Add the soaked and fresh mushrooms, along with the carrots, cabbage and mange touts and continue to stir-fry for 1–2 minutes.
Mix the cornflour and soy sauce together, then add the mixture to the pan along with the bean sprouts and oyster sauce and stir-fry for 1–2 minutes, mixing well. Finally, stir in the sesame oil, then remove the pan from the heat and leave the filling to cool.
Finish the wrappers. Lightly knead the chilled dough for 2 minutes, then divide it into 8 equal pieces. On a lightly floured surface, roll out 1 piece of dough to an 8cm disc. Repeat with another piece of dough so you have two 8cm discs. Sprinkle the top of one of the discs with cornflour, then place the other disc on top. Roll out the stacked discs to a rough 20cm circle.
Heat a non-stick frying pan over a low heat. Place the rolled circle of dough in the pan and cook it for about 30 seconds, until the dough begins to bubble slightly. Immediately turn the disc over and cook it for another 30 seconds, then remove it from the pan. Very carefully peel the dough layers away from each other so you have 2 transparent wrappers. Set them aside and cover them with a damp tea towel or kitchen paper to prevent them from drying out.
Repeat the process until you have 8 wrappers, stacking them on top of each other as you go. When you’re ready to assemble the spring rolls, trim each wrapper into a square.
Assemble the spring rolls. Place about 3 tablespoons of filling 5cm in from a corner of each wrapper. Roll from the corner over the filling once, then fold in each side and then continue the roll until you have completely rolled up the filling inside the wrapper in a neat cylinder. Brush the loose edge with beaten egg to seal. Repeat for all the wrappers.
Fry the spring rolls. Heat a deep-fat fryer to 180°C/350°F, then in batches of about 2 or 3 rolls at a time, use tongs to gently lower the spring rolls into the oil. Fry for 3–4 minutes, turning from time to time, until the spring rolls are golden brown on the outside and cooked through inside. Remove the spring rolls with a slotted spoon and set them aside to drain on kitchen paper. Leave the oil to come back to temperature before frying the next batch.
Make the dipping sauce. Mix all the dipping sauce ingredients together in a jug, then pour them into a serving bowl. Serve alongside the rolls, for dipping.