Stir together all the marinade ingredients in a large bowl. Add the prawns and coat in the marinade; let marinate at room temperature for 20 to 30 minutes.
Preheat the barby to medium-high heat.
Meanwhile, pour all the sauce ingredients - except the peanut oil, mint, coriander and peanuts - into a food processor or blender and process till smooth. While processing, slowly pour in the peanut oil and keep processing until the sauce is smooth and creamy.
Pour into a bowl, stir in mint, coriander and chopped peanuts; season to taste with salt.
Remove the prawns from marinade, shaking off any excess. Cook on the preheated BBQ until pink and firm, about 2 minutes per side. Serve immediately with sauce.