My boyfriend and I love going to Salvadorian restaurants in DC and ordering these hot and cheesy corn cakes with curtido (a condiment resembling coleslaw) and hot sauce. In the restaurants, you see the cook making these very fast, and I'm always amazed by how thin they get the outer layer of dough without any holes. The filling always stays inside, waiting for you!
You have to work with this recipe several times to get it right, but it's worth trying if you want to do your own pupusas. The most important suggestion I have is to always, always use masa harina flour (fine ground cornmeal made from dry hominy). North American corn meal is not an adequate substitute and you'll get an extra greasy, uneven result.
You can find all of the ingredients in a Spanish store or bodega.