Mash the bananas. Mash them thoroughly in a mixing bowl, then add the yoghurt, cumin seeds, baking soda and salt. Add the sugar or jaggery if using. Whisk everything together well.
Begin adding flour. Add ½ cup atta and ½ cup maida first. Mix until the ingredients come together.
Knead a soft dough. Add the remaining flour gradually, alternating between atta and maida as you knead. Aim for a soft dough that you can handle. The moisture in the bananas and yoghurt will decide how much flour you need, so you may use a little less or need a little more than the listed amount. Avoid making it stiff.
Let it rest. Place the dough in a bowl, lightly brush the surface with oil and cover. Rest for 7–8 hours. Plan this step the night before if making buns for breakfast.
Roll thick puris. Divide into portions and roll into small, thick rounds. Use very little flour to keep them from sticking, or lightly oil the rolling surface. Do not roll them thin like regular puris.
Fry one at a time. Heat the frying oil over medium heat. Slide in one puri and gently press the top with a slotted spoon to help it puff. Turn carefully and fry both sides until a deep golden-brown shade. If the outside browns too quickly, lower the heat so the centre has time to cook.
Serve hot. Drain on kitchen paper or a rack and serve with coconut chutney. A cup of strong filter kaapi is a lovely accompaniment.