Preheat oven to 350°F. On a small baking sheet, place garlic drizzled with oil. Roast for 40-45 minutes. Remove the garlic from the oven and set aside, let it cool down.
Increase the oven temperature to 400°. Meanwhile squeeze the garlic cloves from their skins into a medium mixing bowl. Add butter, rosemary, chili flakes, salt and pepper. With a hand held blender (or a food processor,) mix to make smooth paste.
On a rimmed baking sheet, place prawns and slather them with the garlic butter mixture. Cook for 10 minutes until the prawns just curled into a C-shape.
Recipe by Flavor and Friends at http://flavorandfriends.com/garlic-rosemary-prawns/