To make the dressing, place the chillies and salt in a mortar and pound with a pestle. Add sugar and pound for another minute. Place mixture in a small bowl, add mushroom soy sauce, and stir to dissolve. Whisk in the vegetable oil, rice wine vinegar, sesame oil and 2 teaspoons of water until combined. Check seasoning.
Lightly toast the Sichuan peppercorns in a dry saucepan on a high heat for a few minutes, until fragrant, then finely grind using a mortar and pestle.
Peel the potatoes and leave in cold water to prevent discolouration. Slice into matchsticks and place in more cold water.
Boil a pot of salted water. Drain the potato matchsticks and place in boiling water for about 1-2 minutes, until cooked but still holding their shape. Drain, rinse well under cold running water, then drain again. Reserve in a large mixing bowl.
Cut cucumber into matchsticks, discarding the soft centre and its seeds. Add cucumber sticks to the bowl, along with the bean sprouts and coriander.
Whisk the dressing again before adding to the potato mix. Combine well and place on a large share plate. Finish with a drizzle of chilli oil and a pinch of the ground Sichuan pepper. Serve immediately.