Puree first 8 ingredients in a food processor. Taste the mixture and add more sugar, lime juice or chili sauce, if needed. Pour into a pot and simmer 3 to 4 minutes, or until lightly thickened. Cool, transfer to a serving bowl, cover and refrigerate.
Devein prawns by making a shallow cut along the length of the back of each one. Remove any dark material inside. Rinse in cold water, pat dry and set in a bowl. Toss in the oil and salt. Preheat grill to medium-high. Thread a prawn on each skewer. Lightly oil the grill. Grill the prawns 1 to 2 minutes per side, or until cooked through. Arrange on a platter with the sauce alongside.
Note: To peel plums, set into boiling water for 30 seconds, or until their skins loosen. Cool in cold water; peel off the skins.