Pre-soak about 200g chickpeas overnight. The next day drain them and measure out 2 cups (use the rest for hummus or other meals).
Cut the sweet potatoes into 5mm slices, rub them with oil, then either grill them, fry them in a griddle pan or barbecue them. Let them cool, then remove their skins. Mash the flesh with a fork and set aside.
Put the chickpeas in a food processor and pulse for a few seconds (you might need to do this in two batches).
Roughly chop the onion, garlic, coriander leaves and add these to the chickpeas. Add the spices.
Pulse again, until you get a get a coarse texture, resembling fine couscous. You’ll need to scrape the mixture down the sides of the processor occasionally. Add the sweet potato flesh and pulse again.
When you’re happy with the consistency, transfer the mixture into a bowl. (it should stick together - add some chickpea flour, if it doesn't).
Chill for 1-2 hours.
When you’re ready to cook the falafels, take the mixture out of the fridge, add the baking powder and mix well. Using a spoon and your hands, shape small balls or patties. You should get about 16 falafels from the mixture. If they fall apart, add chickpea flour to the mixture.
Put the falafels on a plate.
Pour some vegetable oil into a small, deep saucepan, and heat it over medium heat. When hot, carefully place 5-6 falafels in the oil and fry them for 2-3 minutes, then turn them over and fry until golden brown all over.
Using a slotted spoon, transfer them to a plate, lined with kitchen paper towels.
Repeat the frying with the remaining patties/balls.
Serve at room temperature.