In a small sauté pan over medium heat, melt the butter. Add the garlic and cook, stirring occasionally, until soft, 2 to 3 minutes.
Remove from the heat and add the remaining marinade ingredients. Stir and allow to cool to room temperature.
Using a sharp knife, split open the back of each prawn and devein it. Place the prawns in a large, resealable plastic bag and pour in the marinade. Press the air out of the bag and seal tightly. Turn the bag to distribute the marinade, place in a bowl, and refrigerate for 20 to 30 minutes.
Remove the prawns from the bag and discard the marinade. Grill over direct high heat (230°C to 280°C), with the lid closed as much as possible, until the prawns are just opaque in the center and firm to the touch, 2 to 4 minutes, turning once.
Remove from the grill and serve warm or at room temperature with rice, if desired.