Sift both flours and salt into a large mixing bowl. Add 1 tbsp of the oil and 175-200ml warm water and bring together to make a soft but not too sticky dough. If it feels a little wet, add a bit more flour.
Tip the dough onto a lightly floured worktop and knead for 4-5 mins or until smooth and elastic. Set aside in a lightly oiled bowl for 30 mins.
Divide the dough into six equal-sized pieces (weigh each piece for accuracy if you like). Using a rolling pin roll each one into a thin circle about 18-20cm wide.
Heat a little of the remaining oil and butter in a heavy-based pan over a high heat. Once foaming, add the rolled out roti, one at a time, and cook for 1-2 mins on each side or until speckled golden brown and beginning to puff up. Set aside on a warm plate until ready to eat.