whisk together all the ingredients in a small bowl and refrigerate until ready to use.
Place the prawns and 2 cups of buttermilk into a bowl and toss to coat. Refrigerate for 2 hours.
Preheat oil in a large wok or heavy based saucepan to 180℃.
Combine flour, mustard powder, smoked paprika, crushed fennel seeds, garlic powder and baking powder in a shallow bowl and season well with salt and pepper.
Working with one prawn at a time, lift out of the marinade and toss in the seasoned flour, shaking to remove excess. Then dip in eggwash and once more into the seasoned flour until well coated.
Deep fry in batches for 2-3 minutes or until golden and cooked through. Using a slotted spoon, transfer cooked prawns to a rack set over a baking tray to drain any excess oil.
Place the fennel, chives, dill and chervil in a bowl. Drizzle with buttermilk dressing and toss to combine.
Cut the baguettes lengthways and fill with cos leaves, coleslaw and crispy prawns. Slice to serve.