Wash the potatoes and place them in a saucepan.
Cover with cold water, bring to a boil, and cook for 15-20 minutes until tender.
Drain and lightly mash them, leaving some texture.
Heat 2 tablespoons (30 ml / 1 fl oz) of oil in a large frying pan over medium-high heat.
Once hot, add the mustard seeds.
When they begin to pop, add the cumin seeds, chana dal, and curry leaves.
Stir-fry for one minute until fragrant.
Add the sliced onion, grated ginger, and green chiles to the pan.
Cook for 10-15 minutes, stirring occasionally, until the onions are soft and golden.
Add the mashed potatoes to the pan and stir to combine.
Grate in the coconut cream, add the salt, and mix well.
Stir in the butter or ghee, taste and adjust seasoning if needed, then set the filling aside.
To make the dosa batter, combine the chickpea flour, all-purpose flour, salt, and baking soda in a large bowl. Slowly whisk in 1½ cups (360ml / 12.2 fl oz) of water until you have a smooth, lump-free batter.
Place a 7-8 inch non-stick frying pan over medium-high heat.
Add a teaspoon of oil and swirl to coat.
Pour a ladleful of batter into the center and immediately swirl the pan to create an even, round dosa.
Cook for about one minute, then spread a sixth of the potato mixture in a line down the center of the dosa.
Continue to cook for another 30-60 seconds, until the base is golden and crispy and the filling is heated through. Slide the dosa from the pan, roll it up, and serve immediately.
Repeat with the remaining batter and filling.