How to Make It
Make the nuoc cham In a small bowl, whisk together all of the ingredients.
Make the rolls In a medium, deep baking dish, cover the noodles with boiling water and let stand until softened, about 20 minutes. Drain well and pat dry with paper towels.
Fill a large, shallow bowl with very hot water (not boiling). Soak 1 rice paper wrapper in the water until just pliable, about 30 seconds. Spread on a work surface. Top the wrapper with some of the crab, noodles, cucumber, lettuce, bean sprouts, mint, cilantro and peanuts. Tightly fold the wrapper over the filling, tuck in the sides and roll up halfway. Lay a scallion half across the wrapper with 1 inch of overhang on both sides and tightly roll up. Transfer to a platter and cover with a damp paper towel. Repeat with the remaining wrappers and fillings. Serve the rolls with the nuoc cham and a bowl of hoisin sauce sprinkled with Sriracha and peanuts.