In a shallow, glass dish or bowl, mix together olive oil and melted butter. Stir in lemon juice, mustard, basil and garlic, and season with salt and white pepper. Add prawns, and toss to coat. Cover and refrigerate for 1 hour.
Preheat BBQ to high heat. Remove prawns from marinade, and thread onto skewers. Discard marinade.
Lightly oil BBQ, and arrange skewers on the plate. Cook for 4 minutes, turning once, or until opaque.