Shell prawns carefully, leaving tails intact.
Remove vein with a sharp knife.
Wash prawns under cold running water and dry thoroughly with paper towels.
Place in a deep glass or ceramic bowl.
Combine sambal oelek, garlic and half the oil in a blender.
Blend at high speed until well mixed.
Stir in the remaining oil and salt.
Pour oil mixture over prawns, turning them to make sure they are well coated.
Cover and marinate in the refrigerator for at least 4 hours.
Drain prawns and reserve marinade.
When barbecue coals have burned down to a white ash, set the prawns on the barbecue grill close to the heat.
Cook for 2 to 3 minutes on each side, brushing with reserved marinade, until prawns are firm and pink.
Do not overcook.