For the batter, sift the flour and soda together into a medium bowl. Make a well in the centre and add the egg and Guinness. Whisk from the centre outwards, until just combined. Allow the batter to stand for a few minutes in a cool place.
Heat the canola oil in a deep heavy-bottomed pan until a breadcrumb sizzles and turns brown when dropped into it.
Dip the prawns into the batter and carefully add to the hot oil to deep-fry in batches for 2-3 minutes, or until crisp and golden. Carefully remove with a slotted spoon and drain on kitchen paper.
To make the chilli aioli crack the eggs yolks into a bowl, along with the teaspoon of dijon mustard, and lightly whisk. Whisk the oil into the egg yolks slowly. The oil will begin to get thick and creamy. Stir in the white wine vinegar, garlic and chilli flakes. Season with sea salt and freshly ground black pepper.
Serve the chilli aioli with the Guinness prawn tempura along with the rosemary sea salt.