Whisk the marinade together and toss in the prawns. Coat well and refrigerate for 20 mins.
In the meantime, make the sauce by melting the butter in a pan over medium heat. Add the shallots, ginger and garlic and sauté for a few mins. Add the passionfruit vinaigrette and bring to a simmer. Let cook for about 5 mins.
Remove the prawns from the marinade and skewer them. Place on a perforated grill pan.
Grill over direct medium heat and cook for 2-3 mins on each side until cooked through (pink in colour). Remove from the grill and pour over the sauce. Serve with crispy tortillas and a handful of rocket salad.