Heat a wok until smoking and add the groundnut oil, then add the chicken and mushrooms and stir-fry for 1-2 minutes. Season with soy sauce and five-spice powder, then remove from the wok and set aside to cool for ten minutes. Wipe the wok clean with kitchen paper.
Place the grated ginger, beansprouts, spring onions and carrot into a bowl, then add the chicken and mushrooms and season with the oyster sauce, soy sauce, sea salt and ground white pepper. Stir well to combine.
Lay two spring roll wrappers one on top of the other (the extra thickness will prevent the skin from breaking) and position so that the corners are pointing upwards, downwards and to the sides.
Spoon two tablespoons of the filling into the centre of the wrapper. Brush each corner of the wrapper with the cornflour paste, then bring the two opposite corners on the sides together and, keeping your finger on those edges, bring the bottom corner towards the middle and then roll the pastry up tightly towards the top corner. Tuck the top edge in and seal it with a little of the beaten egg yolk. Repeat with the remaining spring roll wrappers and filling.
Heat the wok over a high heat and fill the wok to a quarter of its depth with groundnut oil. Heat until a cube of bread dropped in sizzles and turns golden-brown in 15 seconds. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Carefully lower the spring rolls in small batches into the oil and deep-fry for 2-3 minutes, or until golden-brown. Remove with a slotted spoon and drain on kitchen paper.
Transfer the spring rolls onto a plate and serve immediately.