Mix the oil with all the other ingredients up to and including the salt, transfer to 2 sheets of aluminium foil. Place the potatoes face down on top, wrap well in the foil.
Charcoal/gas/electric barbecue: Cover and grill the potatoes over/on a medium heat (approx. 200 °C for approx. 15 mins. on each side. Remove the potatoes from the grill, increase the heat to 240°C.
Allow the potatoes to cool, scoop out the flesh with a spoon, puree with ½ of the lime zest, the lime juice and the sour single cream.
Pierce the prawns in the middle with a toothpick, thread the prawns onto the lemongrass skewers, season with salt and the remainder of the lime zest. Grill the prawns over/on a very high heat (approx. 240 °C for approx. 2 mins. on each side, serve with the dip.