For the water dough, whisk together the flour, sugar and salt in a medium bowl. Stir in the oil. Gradually add enough water to form a slightly sticky dough. Knead until smooth. Wrap in plastic wrap and set aside until required. For the butter dough, place the flour in a small bowl. Rub the butter into the flour until it forms a wet sand texture. Add the cold water and stir until a slightly sticky dough forms. Lightly shape the dough into a ball. Wrap in plastic wrap and set aside in the fridge for at least 15 minutes. Meanwhile, cook the potato in a large saucepan of boiling water until tender. Drain and set aside. Heat the oil in a large frying pan. Add the onion and a pinch of salt and cook, stirring often, for 5 minutes or until softened. Add the garlic and coriander root and stir for 1 minute or until aromatic. Stir in the curry powder. Add the potato and stir to coat. Add 1/3 cup water and cook, stirring often, until the water evaporates. Stir in the vegetables, soy sauce, sugar, salt, white pepper and coriander leaves. Set aside to cool completely. To finish the pastry, take the butter dough out of the fridge about 10 minutes before you’re ready to roll out the doughs. Then roll the water dough into a dish approximately 20cm in diameter. Place the ball of butter dough into the centre of the water dough disc and wrap the disc of pastry around the dough ball. Shape into a tight ball and then flatten into a disc. Cover with plastic wrap and refrigerate for 10 minutes. Take the dough out of the fridge and roll it out into a rectangle about 3cm thick. Starting at the short end, roll the pastry up tightly. Use your hands to flatten the cylinder and roll again into a rectangle about 3cm thick. Starting at the short end, roll the pastry up again to form a tight cylinder. Slice into 20 rounds. Roll each piece of pastry into an oval about 5mm thick. Place a heaped tablespoon of filling into the centre. Fold the dough in half and fold the edges over to crimp and seal your curry puff. Transfer to a tray and cover with a damp tea towel to prevent the puff from drying out. Fill a saucepan or wok to about 1/3 capacity with the vegetable oil. Heat over medium heat. Once the oil is hot (165°C/325°F, or when a wooden spoon handle dipped into the oil forms small bubbles), cook the curry puffs, in batches, for about 3 minutes or until golden. Transfer to a tray lined with paper towel. Serve with sweet chilli sauce.