Chop the prawns into small bite-sized pieces. Marinate with salt, white pepper, sugar, fish sauce, light soy sauce, and cornflour. Set aside.
Mix the sauce ingredients in a small bowl and set aside.
Blitz the chillies, garlic, shallots, and 2/3 of the scallion in a blender until finely chopped but still textured.
Heat 2 tbsp of groundnut oil in a wok over medium heat. Add the blitzed aromatics and stir-fry for 2 minutes until fragrant, taking care not to burn the garlic.
Add the remaining chillies and 2/3 of the scallions. Stir-fry for 1 minute.
Add the marinated prawns and stir-fry for 3 minutes until cooked through.
Add the Thai basil leaves, water chestnuts, and prepared sauce. Stir-fry for 1–2 minutes until the basil wilts and everything is well coated.
To serve, dish the prawn mixture into a bowl placed in the centre of a large plate. Surround with baby gem lettuce leaves. Garnish with the reserved scallions and sliced red chilli.