Preheat the oven to 375 degrees F. Combine all ingredients, except buns, butter and lettuce, in a bowl. Split the buns and butter the cut sides. Bake the puns on a baking sheet 8-10 minutes, until lightly browned. Set a lettuce leaf in each bun, and then spoon in prawn mixture. Serve immediately.
Note: Cooked prawns are available in the seafood department. Recipe can be halved.