To make Marinade/Dipping Sauce: In small saucepan over high heat, mix soy sauce and sugar. Bring to boil, stirring. Reduce heat; simmer 5 minutes or until thickened. (Sauce will thicken as it cools.) Mix in lemon juice, to taste; cool. Remove half to bowl; cover and reserve. Add prawns to remaining Marinade/Dipping Sauce; cover and refrigerate 1 to 4 hours.
In bowl, cover noodles with boiling water; soak 5 to 8 minutes until noodles are tender but still firm. Drain, rinse and pat dry; cover and reserve. Drain and grill prawns until cooked throughout. Halve lengthwise; cover and set aside.
Dip 1 rice paper circle in warm water 15 to 20 seconds until pliable. Drain and place on towel. On bottom of paper, place 3 prawn halves and 1 lettuce-leaf half, leaving 3/4 inch uncovered on each side. Top with 1 1/2 Tbsp. noodles, 1 1/2 Tbsp. cucumber and 3 mint leaves. Tightly roll up from bottom, folding in sides half way through. Continue rolling and place seam side down on parchment-lined sheet pan. Keep covered. Repeat with remaining rice papers and filling. Salad rolls should be served within 3 hours of making.
For each serving to order: Halve 2 rolls on the diagonal. Serve with small ramekin Marinade/Dipping Sauce.
Adapted from Chef Patricia Croze, E&O Trading Company, San Francisco, CA