Make sauce: Trim tough ends and outer layers from lemongrass until very tender parts remain, then coarsely chop. Whirl lemongrass in a food processor until minced. Scrape into a bowl and stir in 1/4 cup water and remaining sauce ingredients.
Make bundles: Lay shiso leaves flat on a work surface, veined side up. Set a cilantro sprig across base of each leaf; then set a shrimp on top, with tail poking out the side.
Fold shiso over the rest of shrimp. Roll up bundles from base and secure each with a toothpick. Serve with dipping sauce.
* Grow your own, or find at Asian markets and specialty produce stores.