Cut each prosciutto slice across the width to give four even-sized strips and then in half lengthways to give eight pieces each and 16 in total, one for each prawn. Wrap each piece of prosciutto around the centre of each prawn like a blanket. Make a prawn into a C shape and push the bottom end of a rosemary stick in through the top of the ‘C’ right through to the bottom of the ‘C’. Push the prawn about halfway up the stick and then add another prawn in the same way. Repeat to make eight skewers in total.
Get a griddle pan hot. Brush the prawns with a little oil, season with salt and pepper and then, working in batches if necessary, griddle them for about 1-2 minutes each side depending on their size, until cooked through. Keep each batch on a warm plate covered with foil as you cook the next.