Remove the shells from the prawns, leaving the tail intact. Make an incision with a sharp knife down the back of each prawn. Rinse out the vein from the prawn. With a sharp skewer, make a hole through the centre of the prawn, from top to tail.
Sprinkle the prawns with the garlic salt.
Thread two carrot batons through the hole you have made in the back of the prawns. (The carrot strips should be poking out of the top end of the prawns.)
Place the prawns and the buttermilk into a bowl and leave to soak for 30 minutes, stirring occasionally.
Heat the oil in a deep heavy-based pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Meanwhile, sieve the flour, polenta, chilli powder, salt and freshly ground black pepper onto a large plate.
Remove a prawn from the buttermilk (make sure there's lots of buttermilk on it).
Dredge the prawn in the spiced flour and polenta mixture to coat. Don't shake off the excess.
Repeat with five more prawns, then place the coated prawns carefully into the hot oil. Deep-fry for about 2-4 minutes, turning the prawns occasionally to keep them moving, until crisp and golden-brown all over, then remove with a slotted spoon and drain onto kitchen paper.
Repeat this process of dredging and deep-frying the remaining six prawns. (The tail of the shrimp should be protruding from one end and the carrot from the other - to resemble a firecracker.)
For the dipping sauce, mix the mayonnaise and hot chilli sauce together in a bowl.
To serve, place the bowl of dipping sauce into the centre of a large serving plate. Arrange the 'firecracker' prawns around the bowl and serve.