Preheat oven to 200°C. Prick the whole sweet potatoes with a fork and put them on a plate. Microwave on high for about ten to 15 minutes until quite soft (alternatively put the potatoes in a 220°C oven and bake for 25 minutes, remove them from the oven and reduce the heat to 200°C). Remove and set aside to cool down slightly. Score the skins with a sharp knife.
Peel the ginger by scraping off the skin with a spoon. Peel the garlic. Rip the stem from the chilli, cut it in half lengthways and remove the seeds if you prefer a milder flavour. Cut the lime in half and squeeze the juice into the food processor, catching any pips in your other hand. Put all the rest of the curry paste ingredients into the food processor and whizz to a thick paste.
Cut two squares of foil big enough to fully wrap your sweet potatoes in and grease one side with oil. Take half the curry paste and use your hands to encase one of the potatoes with a thick layer of paste. Repeat with the second potato.
Tightly wrap the sweet potatoes in the foil squares, put on the baking tray and bake in the preheated oven for 30 minutes. Take the sweet potatoes out of the oven, remove the foil and serve with a small side salad and a big spoonful of guacamole, if using.