In a large frypan over medium heat, cook the garlic in the olive oil until just golden, approximately 2 minutes. Be careful not to burn the garlic. Add the tomatoes and sprinkle with salt and cayenne pepper. Bring to a gentle simmer and cook, stirring occasionally, for 15 minutes.
Season the prawns with salt and add to the tomatoes. Cook 3 to 5 minutes, or until no longer pink. Stir in 1 tablespoon of butter for added richness, if desired.