Make the dipping sauce: whisk all sauce ingredients together until smooth. Transfer to a bowl and top with crushed peanuts and chilli flakes.
Prepare all the filling ingredients and arrange on a work surface.
Fill a wide shallow bowl with warm water. Dip one rice paper wrapper into the water for 10–15 seconds until just pliable — do not over-soak.
Lay the wrapper flat on a clean, damp cloth. In the lower third, arrange a few prawns (pink side down), followed by a leaf of lettuce, vermicelli, mint, coriander, carrot and cucumber.
Fold the bottom of the wrapper up over the filling. Fold in the sides, then roll firmly to the top.
Repeat with remaining wrappers. Serve with the peanut dipping sauce.
