To make the mayonnaise, combine the water, lemon grass and lime rind in a saucepan over high heat. Bring to the boil. Cook for 5 minutes or until liquid has reduced to 1 tablespoonful. Set aside for 15 minutes to cool.
Strain the lemon-grass mixture through a fine sieve into a small bowl. Add the mayonnaise and basil and stir to combine.
Thread the prawns, tail-end first, onto the bamboo skewers.
Preheat a barbecue grill on high. Combine the Sichuan pepper, black pepper and salt in a bowl. Sprinkle the pepper mixture over the prawns and cook on grill for 1-2 minutes each side or until prawns change colour. Transfer to a serving platter. Serve with lemon-grass mayonnaise and lime wedges.