Combine buttermilk and half the cajun spice in a ceramic, glass or stainless steel bowl, add the prawns and toss to coat. Allow to marinate in the fridge for at least 2 hours.
Combine the mayonnaise, seeded mustard and lemon juice. Season with salt and pepper to taste and put aside.
In a separate bowl, combine the Panko breadcrumbs, flour and remaining cajun blend.
Heat oil in a deep fryer, wok or saucepan to 180°C. Drain the prawns from the spiced buttermilk mixture and toss in the breadcrumbs to completely coat. Allow the prawns to sit in the crumbs, or on a separate plate, for about 5 minutes.
Fry the prawns until golden brown – about 4 minutes – then drain on kitchen paper and sprinkle with a little Kosher Salt. Split the baguette lengthways and place the prawns down the roll.
Drizzle with a little of the mayo, a little hot sauce (optional), add a handful of shredded lettuce and eat immediately, with plenty of serviettes and a cold beer.