Start by butterflying the prawns. Using a small serrated knife, cut down the back of the prawns through the shells and the flesh from the heads to the tails, cutting just far enough that you can open it up, then remove the black entrails. Once all the prawns are butterflied, set aside.
Combine all the salsa ingredients in a bowl with some seasoning. Ideally leave the salsa at room temperature for 30 mins so that the flavours can infuse.
Mix all the seasoning ingredients together, along with 1 tsp fine salt and 1/ 2 tsp cracked pepper, in a bowl. Brush the prawns inside and out with the melted butter, then toss in the spice mix. Heat two large frying pans over a high heat, tip in the prawns and cook for 4-6 mins until they have just turned pink and the insides are opaque. Serve on a board with the salsa and lime wedges.