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Blanch the prawns in a pot of boiling water for 1 to 2 minutes, or until pink and opaque. Remove immediately, dunk into ice water to stop cooking, drain, and cool in the refrigerator. Once cool, remove the shells and halve each prawn lengthwise.
Bring a pot of water to a boil. Add the rice noodles and cook until tender but not mushy, about 6 minutes. Drain and cool under cold running water. Refrigerate until ready to use.
Remove any tough veins from the center of the lettuce leaves and discard. Place the carrots, bell pepper, and green onions in individual bowls.
In a large bowl of cold water, dip a rice paper for about 10 seconds; gently shake off excess water and lay the paper flat on a clean damp kitchen towel. Stack several lettuce leaves on the lower third of the rice paper. Top with 2 to 3 tablespoons of noodles, 2 prawn halves, a few carrots, bell peppers, and green onions, and finally 2 mint leaves and 2 cilantro sprigs.
Fold the bottom edge of the rice paper up over the filling. Fold in the sides, then roll up into a tight cylinder (like a burrito) to seal.
Repeat the assembly process with the remaining