To prepare rolls: Cook shrimp in boiling water just until curled and opaque in the center, 1 to 2 minutes. Drain and refresh under cold running water. Transfer to a clean cutting board to cool, then chop.
Combine the shrimp with sprouts, scallions, mint, 1 tablespoon fish sauce and 1 tablespoon lime juice in a large bowl. Working with one wrapper at a time, soak in a large bowl of very warm water until softened, about 30 seconds. Place the soaked wrapper on a clean, damp kitchen towel. Put a generous 1/3 cup of the shrimp filling in the center, fold the wrapper over the filling and roll into a tight cylinder, folding in the sides as you go. Repeat with the remaining wrappers and filling.
To prepare dipping sauce: Stir together 3 tablespoons lime juice, water, 2 tablespoons fish sauce, sugar, garlic, serrano (or crushed red pepper) and scallion (if using) in a small bowl until the sugar is dissolved.
To serve, cut each roll in half and serve with the dipping sauce.
Make Ahead Tip: Prepare rolls; cover with a damp kitchen towel and plastic wrap and refrigerate for up to 2 hours.
Shrimp is usually sold by the number needed to make one pound. For example, “21-25 count” means there will be 21 to 25 shrimp in a pound. Size names, such as “large” or “extra large,” are not standardized, so to get the size you want, order by the count per pound. Both wild-caught and farm-raised shrimp can damage the surrounding ecosystems when not managed properly. Fortunately, it is possible to buy shrimp that have been raised or caught with sound environmental practices. Look for fresh or frozen shrimp certified by an independent agency, such as the Marine Stewardship Council. If you can’t find certified shrimp, choose wild-caught shrimp from North America–it’s more likely to be sustainably caught.
Rice-paper wrappers are translucent, round sheets made from rice flour. They need to briefly soak in warm water to make them soft and pliable before using. Find them in the Asian section of large supermarkets or at Asian food stores.