Parboil the potatoes in a large pan of lightly salted boiling water for 5-7 minutes. Drain and allow to cool, then peel off the skins and pierce all over with a fork.
Put the chilli powder in a bowl, pour in 750 ml / 1 1/4 pints (3 1/4 cups) water and mix together. Set aside.
Heat the oil in a large, heavy-based frying pan (skillet) over very low heat, add the potatoes and fry for at least 45 minutes, or until crisp and golden brown on the outside but soft inside.
Remove from the pan with a slotted spoon and drain on kitchen paper (paper towels).
In the same pan, heat about 6 tablespoons of oil over medium heat, add the whole spices and stir-fry for 1-2 minutes, or until they start to splutter, then add the ground spices and stir-fry briskly for 1 minute.
Add the yoghurt and cook for a further 2 minutes. Add the chilli water and potatoes and season with salt. Simmer over very low heat for about 10 minutes, or until the potatoes are done.