Remove the antennae and legs of the prawns.
Slit the back of the prawns, devein and open.
In a skillet, heat butter and saute garlic and ginger for a few minutes. Set aside.
Lay out the aluminum foil on a baking dish.
Put prawns on the foil and season with salt and pepper.
Drizzle with calamansi juice, melted butter mixture and soy sauce.
Top with spring onions and seal both ends of the aluminum foil.
Bake in a 350 °F oven for 15 minutes or until cooked.