Whisk the flour, rice flour, cornstarch, turmeric, baking powder, salt, and pepper in a large bowl. Beat in the egg, then pour in the cold sparkling water and whisk just until combined. A few lumps are fine.

The batter should be the consistency of thin pancake batter and run off a spoon in a ribbon. Refrigerate for 15 minutes.

Peel the sweet potato and slice into ¼ inch planks. Stack the planks and cut into ¼ inch matchsticks about 3 inches long. Do not grate. Hold the matchsticks in cold water as you work.

Toss the shrimp with the fish sauce, minced shallot, minced garlic, and pepper. Let stand 10 minutes.

Drain the sweet potato matchsticks and dry them thoroughly on a clean towel just before frying.

Heat 2 inches of oil to 350°F in a deep pot or wok. Set a wire rack nearby.

Toss a large handful of dry matchsticks with about 3 tablespoons of batter, just enough to glue them together. Pile into a shallow ladle, flatten into a rough 4 inch disc, and lay one or two shrimp on top.

Lower the ladle into the oil, hold for 10 seconds, then tip gently so the fritter releases in one piece. Fry 4 to 5 minutes, flipping once, until deep golden and crisp.

Lift onto the wire rack and season with a small pinch of salt. Fry no more than 2 fritters at a time, letting the oil return to 350°F between batches.

Stir the sugar into the warm water until dissolved. Add the fish sauce, lime juice, minced garlic, and sliced chile. Taste and thin with a little water if needed.

Serve the fritters with lettuce leaves, mint, and perilla. Break off a piece of fritter with shrimp, wrap it in a lettuce leaf with herbs, and dip in the nuoc cham.