Peel the sweet potatoes and cut them into batons, 3-4 inches long and 1/4 inch thick. Add the cut potatoes into a large bowl filled with iced water and let it stay in the fridge for 1 hour to crisp up. Drain well when ready to use.
Whisk together the flour, cornstarch, turmeric and 1/2 tsp salt. Make a well in the centre. Gradually add enough water, whisking constantly, to form a smooth and thick batter. Let the batter rest for 30 minutes.
Preheat the oven to 275 degrees F.
Place the prawns in a bowl and season with remaining salt and pepper. Stir and allow to stand.
Divide the potatoes into 1 ounce portions. You can also divide them based on how big you want the fritter to be but less than the size of your palm is best for frying.
Heat the oil in a large saucepan over medium-high heat until it reaches 350 degrees F on a kitchen thermometer.
Add the sweet potatoes and prawn to the batter and toss well to coat. Spoon each portion of the fritter into the heated oil and cook for 3-4 minutes on each side, until golden and crispy.
Use a slotted spoon to remove them and drain on a plate lined with a paper towel. Repeat with the remaining portions until you’re finished frying.
Transfer the fried fritters to the oven to keep warm.
To make the dipping sauce, combine everything in a bowl and whisk together until combined.
When ready to serve, place the herbs and fritters on a large serving platter. Wrap the fritter and a few herbs in the lettuce and dip it into the dipping sauce.