Combine the sauce ingredients in a small bowl. Taste for preference and adjust quantities if desired.
Pat the prawns dry to remove excess moisture.
In a large mixing bowl whisk the flour, cornflour and all of the seasoning besides 1/2 tsp salt and 1/4 tsp pepper. In a large shallow dish next to the bowl combine the Panko with the leftover salt and pepper.
Add the prawns to the large bowl of flour and toss to coat, then shake and remove, leaving the excess flour in the bowl. Whisk in the egg and enough buttermilk to create a thick(ish) batter. I find different brands of buttermilk have different consistencies so use your best judgement. It should be able to comfortably coat the back of a spoon, but not so thick it's sludgy and firm to mix.
Use one hand to take a prawn and coat it in the batter (hold it by the tail), give it a shake then add to the Panko. Use your other hand to coat it in the breadcrumbs, then place it on a tray. Repeat with the remaining prawns.
Add enough oil to comfortably coat the base of a heavy-bottom pan and place it over medium-high heat. Once the oil hits 180C/350F (a crumb should rapidly sizzle) add half of the prawns (don't overcrowd the pan). Cook for around 3 minutes, turning once halfway, or until golden, crisp and cooked through the centre. Remove and place on a wire rack above the tray, then repeat. They won't take long so just be vigilant they don't overcook.
If you're making the tacos toss the cabbage in a good squeeze of lime juice (work to taste) then add to the tortillas with avocado, prawns, a drizzle of sauce and coriander if using.