In the food processor bowl with the metal blade inserted (or in the blender) , blitz the bananas, yoghurt, sugar and salt to a smooth puree.
If using the food processor, change to the plastic kneading blade and add most (about 2 cups) of the flour and pulse gently until combined into a dough. It should form a firm, slightly sticky dough. Add the remaining flour if needed and pulse again to obtain a dough as described. Whole wheat flour will absorb more liquid and yoghurt consistencies vary, so the amount of flour suggested is only a guide.
At this point, you can transfer the dough to a bowl, cover and leave it at room temperature overnight. I like to leave it in the food processor bowl with the feeding tube covered. This way, I can easily & conveniently adjust the consistency of the dough the next morning if I need to.
Next morning, the dough will feel slightly looser than when you left it. It will be sticky but if you feel it's too tricky to roll out, add flour a tbsp or two at a time.
Heat oil for deep frying. Meanwhile, make around 1" balls (about 30 - 40g each) of the dough. You might need to grease your hands with a bit of oil to handle the dough and to roll it out. Do not use flour to roll out. Dip each ball in a tiny bit oil and you should be able to roll each one out to a small disc of about 3 - 4 inches in diameter.
Deep-fry at medium heat until golden and puffed up. Serve hot with a coconut chutney or anything else you fancy.