First, a word on this dish
Here is something that surprises people. The batter for dosa is nearly the same as for idli, yet what you do with it after fermentation takes a completely different skill. You ladle batter onto a hot griddle, spread it into a wide, thin, even circle, and get the edges crisp while the middle turns golden, all within the time the dosa takes to cook. Few recipes in this library depend as much on technique. Your batter can be perfect and the dosa can still fail if the pan is at the wrong temperature or your spreading hand hesitates. When it works, you get a dosa that is thin and crisp with a light tang from fermentation. You can fill it with spiced potato, as in masala dosa, or eat it plain with chutney and sambar.
The story behind the dish
Where dosa first appeared is as unsettled as the origin of idli, because both belong to one fermented-batter tradition in South India and the surviving evidence for either is thin. What we do know is that dosa has long been a pillar of eating in Tamil Nadu and Karnataka, each place developing a style of its own. Restaurants around Chennai are known for huge, paper-thin dosas, while parts of rural Tamil Nadu and Karnataka prefer a thicker, rougher one. Udupi, on the Karnataka coast, usually gets the credit for making masala dosa (the one wrapped around spiced potato) famous, and Udupi-style restaurants later carried that format across India and overseas.
The method, in order
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Prep
Loosen the batter
Before cooking, check the batter. For dosa it needs to be thinner than for idli, about the thickness of a rich pancake batter that runs off the spoon but still feels substantial. Add water one spoonful at a time. How much you need depends on how long the batter has fermented and how thick it was when you started.
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Prep
Heat the griddle and grease it
Place the griddle over medium-high heat and wait until a few drops of water dance and disappear almost immediately. Then dip half an onion in oil, or use an oiled cloth, and rub the whole surface. This seasons the pan so the first dosa lifts off without sticking, and that first one is the one that usually gives trouble.
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Cook
Pour and spread in one smooth motion
Lower the heat to medium. Pour a ladle of batter in the middle of the pan and, straight away, use the back of the ladle to draw it outward in a quick spiral until you have a thin, level circle. Any pause midway shows up as thick spots, so make up your mind before you start and finish the movement in one go.
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Cook
Drizzle oil and cook until crisp
Spoon a thin trickle of oil or ghee around the rim and over the top. Now leave it for 1 1/2 to 2 minutes, until the edges curl up, turn golden-brown, and the surface looks dry and set. Do not flip too soon. For a thin crisp dosa, you often do not flip at all.
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Cook
Fold or fill, then lift it off
Check that the bottom is a rich golden and crunchy, then slide a flat spatula under it and fold it in half or into thirds. For masala dosa, lay the potato filling down the centre before folding. As you work through the batch, lower the heat if the pan overheats and raise it if the batter will not spread or crisp as quickly as it should.
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Serve
Serve at once
A dosa is at its best right off the griddle, while it still crackles, and it softens within a few minutes. Bring it to the table with coconut chutney and sambar.
Ingredient stand-ins
Batter that has only fermented for a day gives a softer, milder dosa, and batter that is 2 to 3 days old gives a tangier and usually crispier one. Either is correct. Some cooks blend a spoon of cooked rice or a pinch of sugar into the batter for extra crispness, a habit that is common, though not traditional everywhere.
Making it milder or fiercer
The dosa itself is not spiced. Whatever heat and flavour you want comes from the coconut chutney, the sambar or a seasoned filling such as masala dosa's potato, and you can tune each of those on their own.
The slip most cooks make
The temperature of the pan accounts for most failures. Too cool, and the batter will not spread thin or crisp up, so it clings and rips. Too hot, and it sets on contact before you can move the ladle, leaving a thick, lumpy dosa. A hesitant hand is the next cause, since stopping partway through the spiral leaves heavy patches. Last, many people forget to re-grease the pan between the first few dosas, and the early ones stick as a result. Usually that fades after two or three, once the surface has built up a thin film.
Storing what is left
Fridge or counter: In the fridge, the batter keeps fermenting very slowly and stays usable for 4 to 5 days, turning more sour each day. It thickens as it sits, so thin it with water before every cooking session. Finished dosas neither keep nor reheat well, as their charm lies in being fresh and crunchy.
Bringing it back to life: If you must reheat a cooked dosa, a minute on each side in a dry pan over medium heat will bring back part of the crispness, but it will never match a fresh one. This is a dish to cook to order and not to make ahead.
Handle with care
Properly fermented dosa batter smells pleasantly and mildly sour, as idli batter does. If yours smells sharp, alcoholic or rotten, or shows any discolouration or mould, discard it and do not cook with it. Once fermentation has reached the stage you want, keep the batter in the fridge between uses and bring only the portion you plan to cook to room temperature for a short while, which makes it easier to spread.