1 Prepare the nuoc cham sauce: In a bowl, mix 1 tablespoon of sugar, 1 1/2 tablespoons of lime juice, and the water.
Taste the limeade mixture; if needed, adjust with the remaining 1 teaspoon sugar and/or 1 1/2 teaspoons lime juice. If it becomes too strong, add water to balance. If you notice an unpleasant tart-bitter edge, incorporate the vinegar to smooth things out.
Finally, stir in the fish sauce to give it a bold, finishing touch. Optionally, add the chiles and/or garlic. Set aside. This yields 1/2 cup of nuoc cham, which can be prepared and stored in the fridge up to 8 hours ahead.
2 Salt and squeeze the sweet potato and carrot: In a large bowl, toss together the grated sweet potato, carrot, and salt. Use both hands to massage the mixture until it becomes wet and squishy, about 1 minute. Transfer the grated vegetables to a piece of muslin or a non-terry dish towel, wrap them up, and stand over the sink to firmly squeeze and twist out the moisture.
Place the squeezed veggies into a dry bowl.
3 Combine the pancake batter: Add the green onion and mix with a fork to distribute evenly. Stir in the pepper, fish sauce, cornstarch, and rice flour. Crack the egg into the mixture, break it up with a fork, and combine thoroughly. Press the mixture down to compact it, then divide into 8 portions like a pie, using a 1/4 cup measuring scoop.
4 Fry the pancakes: Heat a large nonstick skillet over medium heat and add about 2 tablespoons of oil to coat the bottom. Fry in two batches, cooking 4 pancakes at a time.
For each pancake, use a fork and your fingers to scoop up one of the 1/4 cup portions (one wedge of the “pie”). Drop it into the skillet, then spread and flatten the mixture with the bottom of the measuring cup and a fork to form a 3 1/2-inch-wide pancake (they often end up looking like Australia).
Fry for about 3 minutes, until the edges are richly brown and crisp. Drizzle 2 to 3 teaspoons of oil over the tops of the pancakes, then use two spatulas to carefully flip each one. (The pancakes are delicate, so two spatulas are very helpful.)
Fry for another 3 minutes or so, until the underside is crisp and brown. Transfer to a rack to cool. Repeat with the remaining batch.
5 Assemble and serve the lettuce wraps: Serve with the lettuce, herbs, and sauce. Let guests build their own lettuce leaf wraps, placing a piece of pancake inside (use scissors or forks/fingers to break the pancakes into smaller pieces suitable for wrapping), along with 2 or 3 herb leaves and/or torn cilantro sprigs. Dip in the sauce and enjoy. These are best eaten the day they are made.
