Place the buttermilk, Dijon mustard, white balsamic and chives in a bowl and whisk to combine.
Preheat oven to 160ºC.
Brush the bread with olive oil and sprinkle with Parmesan on both sides, pressing down to ensure the Parmesan sticks.
Peel the prawns, leaving the tails intact.
Place the bread, prosciutto and tomatoes on baking trays lined with non-stick paper and bake for 12 minutes or until golden and tomatoes are juicy. Set aside to cool.
Place wedges of cos lettuce on four serving plates, top with prawns, prosciutto, tomatoes and Parmesan croutons. Spoon over buttermilk dressing to serve.