How to Make It
In a small bowl, whisk together fish sauce, lime juice, sugar and 2 Tbsp. water until sugar has dissolved. To make the filling, combine chicken, mangoes, cucumber, mint, scallions and cilantro in a large bowl.
Fill a baking pan halfway with cold water. Lay a clean kitchen towel on countertop. Dip 1 rice-paper wrapper in water until pliable, 15 to 20 seconds. (Note: Look for rice paper wrappers near the Asian ingredients in the supermarket. Buy a few extras in case some tear while you're rolling them.) Remove wrapper and shake off excess water. Lay wrapper flat on towel. Place some filling (about 1/2 cup) in middle of wrapper, leaving a 1-inch border on bottom and sides. Fold 1 inch of bottom and 1 inch of sides toward center of round, then roll up tightly. Place rolls on a platter, seam side down. Cover loosely with plastic wrap. Repeat with remaining rolls and filling ingredients. Serve immediately with dipping sauce on the side.