Place the water and the butter into a pan, bring to a simmer then add the leeks. Season with salt and freshly ground black pepper and gently cook for 5 minutes until the leeks have softened. Place into a mixing bowl, cool slightly then add the remaining butter and the breadcrumbs and mix well.
Cut a pocket in the thick part of the chicken breast, fill with ½ cup of the leek stuffing. Secure the filling, by wrapping the whole breasts with 2-3 slices of prosciutto. Gently fry in olive oil for 4 minutes on each side until golden, then place into a pre heated oven at 180°C for 15-20 minutes. Remove and allow to rest for 5 minutes before serving.