Mix the flours, salt, cardamom and cashewnuts. Add the coconut milk and jaggery and add just enough water to make a batter that's pourable.
Heat a non-stick or cast-iron griddle until a drop of water skitters and evaporates quickly. Using a round-bottomed ladle, pour about 1/3 cup of the batter in the center of the griddle.
Quickly spread the dosa by spiraling the batter outward, using the bottom of the ladle.
You can sprinkle a few drops of oil around the edges of the dosa to get it crispy. This also helps release the dosa from the griddle. But if you'd rather omit the oil, do so by all means.
When the dosa is golden-brown on the underside and comes of the griddle quite easily, flip it over and cook for another 30 seconds or so.
Serve hot. I like mine with a dollop of vegan butter.