To make coriander yoghurt, process half of the yoghurt and the coriander in a food processor until combined. Add the remaining yoghurt and pulse to combine. Chill until required.
To make chapattis, on a clean surface, combine flours, cumin seeds, ground coriander, baking powder and salt. Make a well in the centre and add the extra yoghurt. Mix and knead until a soft dough forms.
Divide dough into 8 equal portions. Lightly flour a clean work surface and roll out each portion to around 20 cm.
Heat a char-grill pan or frying pan over high heat. Spray chapatti with oil. Cook for 1 minute on one side and then turn and cook for a further minute on the other side. Transfer to a large heatproof bowl and cover with a clean tea towel to soften chapatti. Serve with coriander yoghurt.