Make dough: combine mashed potatoes, salt, chilli powder, turmeric, coriander, and 3 tablespoons vegetable oil in a bowl and mix well. Add the whole wheat flour a little at a time, kneading the mixture until it forms a stiff but pliable dough. (If the dough becomes too stiff, add a few drops of water.) Transfer the dough to an oiled bowl, turning to coat; cover with plastic and set aside for 10 minutes.
Preheat a tawa to medium heat. Divide the dough into lemon-sized portions and roll them out on a lightly floured surface to 1/4-inch thickness. Fry the chapatis using up to 1 teaspoon of oil for each side. Cook until brown spots have started to form on the bread and the dough is cooked through. Brush with ghee or melted butter and serve hot.