To make the sauce mix all sauce ingredients together and whisk until smooth. Set aside.
Drain the chickpeas, reserving about 1/4 cup liquid.
Add the chickpeas, sesame seeds, onion, garlic, salt, pepper, cumin, coriander, harissa paste and 2 Tbsp of the chickpea liquid to the food processor and pulse until almost a paste.
Scoop into a bowl and chill for at least one hour.
Stir together the flour and baking powder and mix into the chickpea mixture.
Form the falafels into golf ball sized balls.
Fry in oil, in batches, for 3-4 minutes until browned and crisp.