Place the first 8 ingredients in a food processor and puree. Transfer to a small pot and gently simmer 5 minutes. Remove from the heat. Preheat your grill to medium-high. Toss the prawns with the oil, cayenne, paprika and cumin. Grill, lightly oiling the surface first if prone to sticking, 2 to 3 minutes per side. Serve the prawns with a bowl of the pineapple sauce alongside for dipping.