Soak the black mushrooms in a bowl of warm water for around 30 minutes. Drain and dry them, then chop finely.
Place the rice noodles in a large bowl, cover with warm water and leave to steep for 15 minutes. Cook for 3 minutes in a saucepan of boiling salted water. Drain.
Place the pork, chicken, rice noodles, mushrooms, carrot, garlic, ginger and nuoc-mâm in a large bowl. Season and mix well.
Soften the spring roll wrappers by dipping them briefly in a plate of lightly sugared water, then wipe off the moisture and lay them flat on the work-surface. Place some stuffing on each wrapper, fold the edges in towards the filling and roll up tightly.
Fry the spring rolls in very hot vegetable oil and drain on kitchen paper.
Serve the warm spring rolls with a few salad leaves and a spicy sauce.